Treat whitespace-only FAL_KEY the same as unset so users who export
FAL_KEY=" " (or CI that leaves a blank token) get the expected
'not set' error path instead of a confusing downstream fal_client
failure.
Applied to the two direct FAL_KEY checks in image_generation_tool.py:
image_generate_tool's upfront credential check and check_fal_api_key().
Both keep the existing managed-gateway fallback intact.
Adapted the original whitespace/valid tests to pin the managed gateway
to None so the whitespace assertion exercises the direct-key path
rather than silently relying on gateway absence.
