fix(WaitForProcessing): Shutdown option WaitForProcessing never entered on subscription close

[nico-kn]

As outlined in googleapis#2117, the condition to enter the WaitForProcessing closing behavior of a subscription was checking against an undefined field .isEmpty of the inventory instead of checking for the isEmpty() method of the inventory.

In unit tests this went unnoticed since the field was set accordingly in the class under test.

This implicitly changes the default behavior of the subscription close functionality since the previous PR that introduced this functionality (googleapis#2068) changed the default behavior without effect as the condition was never entered during close().

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Fixes googleapis#2117 🦕

[nico-kn]

Closing in favor of googleapis#2118 on the base repo