Don't raise when there's no bindgen flags

The current implementation expects that `--bindgen-flags` will always
exist if there's one or more flag. However, that's not the case in
reality because `--bindgen-flags` is always there regardless of
whether there's flags or not.

See https://source.chromium.org/chromium/chromium/src/+/main:build/rust/rust_bindgen_generator.gni;l=179-187

Bug: 373864033
Change-Id: I5816d06b7be688946dc5d0186a0852aa9588093c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5994214
Reviewed-by: Etienne Dechamps <edechamps@google.com>
Commit-Queue: Etienne Dechamps <edechamps@google.com>
Auto-Submit: Mohannad Farrag <aymanm@google.com>
Cr-Commit-Position: refs/heads/main@{#1378921}