constant error with 3.1.0 on NO multipart emails

[plandolt]

Description of the problem including expected versus actual behavior:
Constant error on no multipart message regarding not having attachments variable present. In the case the IF on https://github.com/logstash-plugins/logstash-input-imap/blob/master/lib/logstash/inputs/imap.rb#L170 is true then the variable attachments does not get set. Which is causing the error on https://github.com/logstash-plugins/logstash-input-imap/blob/master/lib/logstash/inputs/imap.rb#L213.

Steps to reproduce:

1. use the version logstash-input-imap-3.1.0
2. fetch a NO multipart mail
3. check the error

Provide logs (if relevant):
2021-10-26T22:47:02,514][ERROR][logstash.inputs.imap ][xxx][0a07994cad85ba5ba7c29ee42c2cf8e414ed38435024a0cf3934c6b003696e6e] Encountered error NameError {:message=>"undefined local variable or method attachments' for #<LogStash::Inputs::IMAP:0x18e7c547>", :backtrace=>["/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-input-imap-3.1.0/lib/logstash/inputs/imap.rb:219:in block in parse_mail'", "/usr/share/logstash/logstash-core/lib/logstash/codecs/delegator.rb:64:in block in decode'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-codec-plain-3.0.6/lib/logstash/codecs/plain.rb:35:in decode'", "/usr/share/logstash/logstash-core/lib/logstash/codecs/delegator.rb:62:in block in decode'", "org/logstash/instrument/metrics/AbstractSimpleMetricExt.java:65:in time'", "org/logstash/instrument/metrics/AbstractNamespacedMetricExt.java:64:in time'", "/usr/share/logstash/logstash-core/lib/logstash/codecs/delegator.rb:61:in decode'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-input-imap-3.1.0/lib/logstash/inputs/imap.rb:183:in parse_mail'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-input-imap-3.1.0/lib/logstash/inputs/imap.rb:119:in block in check_mail'", "org/jruby/RubyArray.java:1809:in each'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-input-imap-3.1.0/lib/logstash/inputs/imap.rb:113:in block in check_mail'", "org/jruby/RubyArray.java:1851:in each_slice'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-input-imap-3.1.0/lib/logstash/inputs/imap.rb:111:in check_mail'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-input-imap-3.1.0/lib/logstash/inputs/imap.rb:92:in block in run'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/stud-0.0.23/lib/stud/interval.rb:20:in interval'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-input-imap-3.1.0/lib/logstash/inputs/imap.rb:91:in run'", "/usr/share/logstash/logstash-core/lib/logstash/java_pipeline.rb:405:in inputworker'", "/usr/share/logstash/logstash-core/lib/logstash/java_pipeline.rb:396:in `block in start_input'"]}

Possible workaround
I got it working changing https://github.com/logstash-plugins/logstash-input-imap/blob/master/lib/logstash/inputs/imap.rb#L213 to if defined?(attachments) && attachments.length > 0. But bear in mind: I really do NOT know ruby at all so this might not be the "correct" way to fix it.