cq_deps: Handle commits that don't have Gerrit CLs

Handle rolling commits that don't have associated Gerrit CLs by skipping
them when resolving dependencies.

Change-Id: I68c14df6265b904a7e218d8e53ef5254d87e0c15
Reviewed-on: https://pigweed-review.googlesource.com/c/infra/recipes/+/37460
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Commit-Queue: Marc-Antoine Ruel <maruel@google.com>
Reviewed-by: Marc-Antoine Ruel <maruel@google.com>