SF: Check the active mode for nullptr in dumpsys

We alredy have the check for mSupportedModes.size()==0 which
is supposed to be equivalent to getActiveMode()==nullptr.
However it's better to directly null check getActiveMode()
in case the logic changes in the future.

Fixes: 183381694
Test: dumpsys surfaceflinger
Change-Id: I49bdcb9636e3699bb720a84b6f789790c636b9b5