[Provider Model] Pass null to the reason parameter to avoid unnecessary

bugreports

- The reason parameter may be used to generate bugreport

- Pass null to the reason parameter under normal situation

Bug: 178542910
Test: manual test
- make RunSettingsRoboTests ROBOTEST_FILTER=NetworkProviderSettingsTest

Merged-In: I469a8c72fc7a6ab057acef476296285bb8a62713
Change-Id: I469a8c72fc7a6ab057acef476296285bb8a62713
(cherry picked from commit 3c02095c)