Fix reset field classification service command.

Example command: "cmd autofill set temporary-detection-service 0". The
function needs to call reset the detection service when it finds the
service name is null. Similar to reset augmented autofill.
Otherwise the getNextIntArgRequired() call would
throw error saying the duration is not found in command.

bug:271128589
Test: atest PccFieldClassificationTest, check in log if shell command
"cmd autofill set temporary-detection-service 0" is correctly parsed.

Change-Id: I5937b19e179571567d91808c6f3a74ddc6172815