PackageManager: Avoid null access

In case cache creation fails, mCacheDir may be null. Avoid an
access.

(cherry picked from commit 9f292ce7)

Bug: 74087079
Test: m
Test: boot under low-space condition
Merged-In: I55836c820ef2c3431e8f06224d6f9edf0e9881ef
Change-Id: I55836c820ef2c3431e8f06224d6f9edf0e9881ef