Fix settings to properties mapper

The existing try catch only guards against IOException, while
Nullpointerexception is not guarded. Explicitly handle the null
exception in the code. The return of sendAconfigdRequests can be null
when the aconfigd socket is not on.

Bug: b/406270206
Test: build aosp next food build, and send a local override
Flag: EXEMPT flag infra
Change-Id: If56565822e304fc789d073cdafae2d2bd48873b2