Add UT cases to disallow null thresholds even for privileged apps

Add UT cases to cover the system privileged app with permission
which is allowed to set SystemThresholdReportingRequestedWhileIdle
to true with empty signal strength thresholds but not with null.

Bug: 205585585
Test: atest SignalStrengthUpdateRequestTest
Change-Id: I21631539ea5a068566e143c9418648e04d5a18b9