Change behavior for next() and hasNext() for empty paths

Original behavior treated DONE like any other path verb, and threw
an exception when querying an iterator that had already issued
VERB_DONE. Also, hasNext() returned false after when next() would
result in an exception.

New behavior treats DONE as just informational:
- An empty iterator, or one with no more verbs to iterate on, will
always return VERB_DONE for next()
- hasNext() will only return true when there are non-DONE verbs left to
return from next().

Also, clarified docs for the verbs.

Bug: 264437928
Test: PathTest and PathIteratorTest, including new test for hasNext()
Change-Id: Icc0517ac3c2a346a3271ff2f78e7629e4ebb5d0a