[skottie] Fix spurious gradient error messages

The adapter apply() method can get called before stops resolution, so
only log when some stops are present.

TBR=

Change-Id: I8e336e2cff781a0e64de31e6b63f3cf373b2daa0
Reviewed-on: https://skia-review.googlesource.com/140245
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
1 file changed