Avoid calling some toString() methods directly

Let StringBuilder.append() do that in a way that does not throw any
NullPointerException if the passed object reference happens to be null.

Bug: 406328303
Flag: EXEMPT no functional change
Test: atest DocumentsUIGoogleTests:com.android.documentsui.services
Change-Id: I883d1f24e13678629300a537979830ccc82c1a69