Update many uses of SkFont that had nullptr SkTypeface

A null SkTypeface results in a font that does not draw anything.
These changes are pretty mechanical and use the test SkFontMgr
in benchmarks, tests, and tools, and the fiddle-specific SkFontMgr
(e.g. FontConfig) in our documentation examples.

Change-Id: I3b1fbddf0871c294bde819de75be107bdc849191
Bug: b/305780908
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/787838
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
94 files changed