Fix maxBytes batching sending empty messages

[Legogris]

Fixes #279 and #280.
Regression test added.
Please let me know if you prefer me to split this up in separate PRs.

• Tests and linter pass
• Code coverage does not decrease
• Appropriate docs were updated (not necessary)

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[JustinBeckwith]

Thanks for the PR @Legogris! If you can go through the CLA signing thingy, we'll take a look!

[Legogris]

I signed it!

[callmehiphop]

Looks great, thanks for the PR!

[googlebot]

☹️ Sorry, but only Googlers may change the label cla: yes.

[callmehiphop]

@JustinBeckwith looks like merging is blocked because of the CLA, are there any other steps we need to take?

[googlebot]

A Googler has manually verified that the CLAs look good.

(Googler, please make sure the reason for overriding the CLA status is clearly documented in these comments.)