Two small changes to batch assembly in the pipeline:

• in eventConsumer, don't start assembling batches until we get a non-nil output channel
• in outputController, don't provide eventConsumer with a non-nil output channel if the output list is empty.

The first one prevents batches from being assembled while the output configuration is being updated. The second prevents batches from being assembled when running under Agent if we haven't yet received our output configuration. (When running under Agent, there is typically no output when pipeline.New is called, which previously allowed the construction of batches with no size limit before the output was really initialized.)

Fixes #34703.

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