CTS: Fix an assert error formater in Camera2 tests.

* The boolean condition to insert ", " or not appears to be inverted.
* (Optional) The null check is instrinsic in String + operators, so not needed.

Bug: None
Test: Manually testesd by intentionally adding a failure in \
  ExtendedCameraCharacteristicsTest#testAvailableStreamConfigs

Change-Id: I3b312a19cb804ad25f31a9de047ab8a6a16754b6
1 file changed