Delete invalid nil timestamp event streams

Until ManageIQ/manageiq-providers-kubernetes#501 code
lands, openshift events with nil timestamps will be added to event streams.

These are invalid events and really mess up our queries since we timestamp is a lookup
field and expected to use an index which doesn't happen when it's nil.

db/migrate/20230830134742_delete_nil_timestamp_event_streams.rb

@@ -0,0 +1,11 @@
+class DeleteNilTimestampEventStreams < ActiveRecord::Migration[6.1]
+  class EventStream < ActiveRecord::Base
+    self.inheritance_column = :_type_disabled
+  end
+
+  def up
+    say_with_time("Deleting event_streams with nil timestamp values") do
+      EventStream.where(:timestamp => nil).delete_all
+    end
+  end
+end

spec/migrations/20230830134742_delete_nil_timestamp_event_streams_spec.rb

@@ -0,0 +1,16 @@
+require_migration
+
+describe DeleteNilTimestampEventStreams do
+  let(:event_stream_stub) { migration_stub(:EventStream) }
+
+  migration_context :up do
+    it "removes nil timestamp events, leaving others" do
+      good = event_stream_stub.create!(:timestamp => Time.now.utc)
+      event_stream_stub.create!(:timestamp => nil)
+
+      migrate
+
+      expect(event_stream_stub.pluck(:id, :timestamp).flatten).to eq([good.id, good.timestamp])
+    end
+  end
+end