Prevent pipeline from queueing large batches before an output is set

[faec]

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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• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @faec? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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• Start Time: 2023-03-08T19:03:11.186+0000

• Duration: 73 min 59 sec

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Test	Results
Failed	0
Passed	25991
Skipped	1962
Total	27953

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[rdner]

I think we need tests for the new behaviour.

[faec]

Tests added for both checks. The eventConsumer case required some additional rearranging to make the relevant channel accessible from a test.

[leehinman]

LGTM