Fix bug: can't convert nullptr -> bool

Change-Id: Ib37c46d00b6a3e768e6d8399f2a6afb332d36bff
Reviewed-on: https://skia-review.googlesource.com/5141
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
3 files changed