Fix Nailgun failure when the port is not specified

[Eric-Arellano]

Downstream Twitter tests were failing to compile certain targets with the error message:

zinc[zinc-java](path/to/target:target) failed: an integer is required (got type NoneType)

This happens when ProcessManager.socket returns None, which is an expected return value:

pants/src/python/pants/pantsd/process_manager.py

Lines 306 to 309 in e727747

	@property
	def socket(self):
	"""The running processes socket/port information (or None)."""
	return self._socket or self.read_metadata_by_name(self._name, 'socket', self._socket_type)

This ends up resulting in the nailgun code trying to use None for the port, which is invalid and causes the error.

Instead, we use Pants' idiom of setting the default to None and in the body of the constructor then defaulting to the actual default value we care about.

[ity]

lgtm!

[jsirois]

The commit description seems over the top to me. This is all just the common pattern of using meaningful default param values instead of None plus check in constructor. That central lesson and fix is pretty well obscured by a wall of text.

[Eric-Arellano]

The commit description seems over the top to me.

Agreed, and very good feedback. Because I hadn't yet run the fix through Twitter tests—and it took a while to figure out how all the pieces fit together—I was retracing my steps as an exercise in reasoning about the correctness of the solution. While useful to me, doesn't mean it's helpful in the PR description. I shortened it quite a bit.

Thanks for the review both!