Google3 ClangTidy: cleanup size() calls that a really empty()

The Google3 roller's ClangTidy complained about many uses of size()
where empty() is the better call.

See:
https://critique.corp.google.com/cl/478517696/analysis

Change-Id: I033c55348b59d59c53f489f4d3d61fc057819c02
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/586961
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Herb Derby <herb@google.com>
20 files changed