Allow empty rendertests commandline flags.
Allow empty rendertests commandline flags.

When it's empty, just use default rendertests file.

Bug: skia:10156
Change-Id: I406d381570bbd6b348e6364d649d39af5817a56e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/284523
Reviewed-by: Tyler Denniston <293ae2d31b7e9da32c4fb58bdaaed1f4bef4c6e7@google.com>
Commit-Queue: Tyler Denniston <293ae2d31b7e9da32c4fb58bdaaed1f4bef4c6e7@google.com>
Auto-Submit: Lepton Wu <98fe55dd9d3c1cbbf1dc9faf0be9e8ebf7fc5bb6@chromium.org>

aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia