Add Contract anotation to Strings.isNullOrEmpty

This annotation tells Idea that the method returns true when null is passed
as an argument, removing bogus warnings about potential null access.

Note that this change only adds annotations file. Library descriptor needs
to be updated in a separate change as it resides in a different repository.

Change-Id: I757195c271d4e529e0efe02e25b13a93cd7c80eb
1 file changed