Stanza crashes when using k8s_metadata_decorator operator

[Mayur2021]

Describe the bug
Stanza crashes when using k8s_metadata_decorator operator. Gets the following exception

github.com/observiq/stanza/operator/builtin/transformer/k8smetadata.(*K8sMetadataDecorator).decorateEntryWithNamespaceMetadata(0xc0001e8b40, 0x0, 0x0, 0xc0034753b0, 0x24, 0xc017915f235ea7e8, 0x8bdab987c4, 0x3207fa0, 0xc003844f90, 0xc003844fc0, ...)
/build/cmd/stanza/vendor/github.com/observiq/stanza/operator/builtin/transformer/k8smetadata/k8s_metadata_decorator.go:321 +0x2d0
github.com/observiq/stanza/operator/builtin/transformer/k8smetadata.(*K8sMetadataDecorator).Process(0xc0001e8b40, 0x2164540, 0xc0005d7dc0, 0xc000119400, 0x0, 0xc00348fa10)

To Reproduce
Steps to reproduce the behavior:

1. Build stanza locally and deploy in k8s clustter
2. Used the following configuration

kind: ConfigMap
metadata:
  name: stanza-config
  namespace: stanza-test
apiVersion: v1
data:
  config.yaml: |2-
    pipeline:
      - type: file_input
        include:
          - /var/log/containers/*.log
      - type: json_parser
        id: parser-docker
        output: extract_metadata_from_filepath
        timestamp:
          parse_from: time
          layout: '%Y-%m-%dT%H:%M:%S.%LZ'
      # Extract metadata from file path
      - type: regex_parser
        id: extract_metadata_from_filepath
      #  regex: '^.*\/(?P<namespace>[^_]+)_(?P<pod_name>[^_]+)_(?P<uid>[a-f0-9\-]{36})\/(?P<container_name>[^\._]+)\/(?P<run_id>\d+)\.log$'
        regex: '(?P<pod_name>^.+?)_(?P<namespace>.+?)_(?P<container>.*)-(?P<runid>.+?).log$'
        parse_from: $labels.file_name
        #parse_to: kubernetes
      - type: k8s_metadata_decorator
        namespace_field: namespace
        pod_name_field: pod_name
      - type: otlp_output
        endpoint: opentelemetrycollector.stanza-test.svc.cluster.local:55681
        insecure: true
      #- type: stdout

4. Exception found

	/build/cmd/stanza/vendor/github.com/observiq/stanza/operator/builtin/transformer/k8smetadata/k8s_metadata_decorator.go:321 +0x2d0
github.com/observiq/stanza/operator/builtin/transformer/k8smetadata.(*K8sMetadataDecorator).Process(0xc0001e8b40, 0x2164540, 0xc0005d7dc0, 0xc000119400, 0x0, 0xc00348fa10)
	/build/cmd/stanza/vendor/github.com/observiq/stanza/operator/builtin/transformer/k8smetadata/k8s_metadata_decorator.go:179 +0x20d
github.com/observiq/stanza/operator/helper.(*WriterOperator).Write(0xc00015bce0, 0x2164540, 0xc0005d7dc0, 0xc000119400)
	/build/cmd/stanza/vendor/github.com/observiq/stanza/operator/helper/writer.go:56 +0x10c
github.com/observiq/stanza/operator/helper.(*ParserOperator).ProcessWith(0xc00015bce0, 0x2164540, 0xc0005d7dc0, 0xc000119400, 0xc00348fa98, 0xc003599680, 0x30)
	/build/cmd/stanza/vendor/github.com/observiq/stanza/operator/helper/parser.go:89 +0xfb
github.com/observiq/stanza/operator/builtin/parser/regex.(*RegexParser).Process(0xc00015bce0, 0x2164540, 0xc0005d7dc0, 0xc000119400, 0xc00027bb40, 0x413861)
	/build/cmd/stanza/vendor/github.com/observiq/stanza/operator/builtin/parser/regex/regex.go:77 +0x72
github.com/observiq/stanza/operator/helper.(*WriterOperator).Write(0xc000222300, 0x2164540, 0xc0005d7dc0, 0xc000119400)
	/build/cmd/stanza/vendor/github.com/observiq/stanza/operator/helper/writer.go:56 +0x10c
github.com/observiq/stanza/operator/helper.(*ParserOperator).ProcessWith(0xc000222300, 0x2164540, 0xc0005d7dc0, 0xc000119400, 0xc00027bba8, 0xc00027bbe0, 0x559603)
	/build/cmd/stanza/vendor/github.com/observiq/stanza/operator/helper/parser.go:89 +0xfb
github.com/observiq/stanza/operator/builtin/parser/json.(*JSONParser).Process(0xc000222300, 0x2164540, 0xc0005d7dc0, 0xc000119400, 0x55909a, 0xc000421aa0)
	/build/cmd/stanza/vendor/github.com/observiq/stanza/operator/builtin/parser/json/json.go:52 +0x72
github.com/observiq/stanza/operator/helper.(*WriterOperator).Write(0xc0002db090, 0x2164540, 0xc0005d7dc0, 0xc000119400)
	/build/cmd/stanza/vendor/github.com/observiq/stanza/operator/helper/writer.go:56 +0x10c
github.com/observiq/stanza/operator/builtin/input/file.(*Reader).emit(0xc0037c8d80, 0x2164540, 0xc0005d7dc0, 0xc003830000, 0x8d, 0x4000, 0x0, 0x0)
	/build/cmd/stanza/vendor/github.com/observiq/stanza/operator/builtin/input/file/reader.go:129 +0x399
github.com/observiq/stanza/operator/builtin/input/file.(*Reader).ReadToEnd(0xc0037c8d80, 0x2164540, 0xc0005d7dc0)
	/build/cmd/stanza/vendor/github.com/observiq/stanza/operator/builtin/input/file/reader.go:98 +0x40d
github.com/observiq/stanza/operator/builtin/input/file.(*InputOperator).poll.func1(0xc00057e140, 0x2164540, 0xc0005d7dc0, 0xc0037c8d80)
	/build/cmd/stanza/vendor/github.com/observiq/stanza/operator/builtin/input/file/file.go:149 +0x6b
created by github.com/observiq/stanza/operator/builtin/input/file.(*InputOperator).poll
	/build/cmd/stanza/vendor/github.com/observiq/stanza/operator/builtin/input/file/file.go:147 +0x3c7

Expected behavior
Stanza should not crash, atleast tell there is a config error.

Screenshots
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Environment:

• OS
• Stanza Version or Commit Hash

Additional context
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