allow client to pass null if there are no uniforms

Change-Id: I788574ccd1f66cfa29556d7d84ae813d108a39c7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/271318
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
2 files changed