Fix potential NPE while releasing worker thread of ImageWallpaper

We nullify mWorker right in ImageWallpaper#destroy(),
we might get NPE if postRender is invoked later.
Thus, we add null check to avoid NPE.

Bug: 144039509
Bug: 146167292
Test: atest com.android.systemui
Test: atest CtsMultiUserHostTestCases
Change-Id: I209673bdac7dcfee765006568583b5a9d6037b95
Merged-In: I243274af54538fc89268c448aa2c5a95f63c7ae3