don't clobber existing values

[ab77]

• don't overwrite existing non-null values in the API_KEYS array with null values
• (e.g.)

REGISTRY2_S3_SECRET=""

TOKENS_CONFIG=IMG_S3_SECRET_KEY:hex,S3_MINIO_SECRET_KEY:IMG_S3_SECRET_KEY,S3_MINIO_SECRET_KEY:REGISTRY2_S3_SECRET

API_KEYS[IMG_S3_SECRET_KEY]=<random hex value>
API_KEYS[S3_MINIO_SECRET_KEY]=API_KEYS[IMG_S3_SECRET_KEY]

.. and since REGISTRY2_S3_SECRET is undefined:
API_KEYS[S3_MINIO_SECRET_KEY] is untouched and still contains the value of API_KEYS[IMG_S3_SECRET_KEY]

change-type: patch

[ab77]

lgtm