Agent V2: Check that a stream ID exists before generating an add_fields processor for it

[cmacknz]

In the feature-arch-v2 branch we automatically add a processor to inject the stream ID from the agent policy as event metdata, but we don't check if the stream ID exists before doing this. We should include a check for an empty stream ID before adding the processor.

This is the relevant code in the feature-arch-v2 branch:

beats/x-pack/libbeat/management/generate.go

Lines 193 to 196 in a528bd2

	// source stream
	streamID := streamExpected.GetId()
	sourceStream := generateAddFieldsProcessor(mapstr.M{"stream_id": streamID}, "@metadata")
	processors = append(processors, sourceStream)

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[jlind23]

Closing this per #33335 merge