add SkTHashTable::LookupOrNull()

This should help avoid confusion from T**.

Change-Id: I1851baa2a55714721fa935d234b6a4a1c6d6504f
Reviewed-on: https://skia-review.googlesource.com/c/182562
Commit-Queue: Mike Klein <mtklein@google.com>
Commit-Queue: Herb Derby <herb@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Herb Derby <herb@google.com>