[res] Fix a crash in dumpHistory() when impl==null

setImpl() may sometimes put a null ResourcesImpl object into
Resources (e.g. when we couldn't load the APK), and in that case
the whole bugreport generation fails with a system server crash.

This fix makes it log a "null" string for that Resources object
instead

Bug: 321161602
Test: build + boot + bugreport
Change-Id: Ia955c9433c845e75d3813378220a166e67911182