Add null check for notification preference helper

Not sure about the exact situation, but depending on timing or
vendor implementation, this NPE could happen and at least we
should avoid this as it's system_server process.

We observed this when running WM CTS for ARC.

Bug: 225309857
Test: Run WM CTS
Change-Id: I095b0f51d8a0d436cca5072073032b26ddd11a90