cmd/gitops-pusher: only use OAuth creds if non-empty string

`os.LookupEnv` may return true if the variable is present in
the environment but an empty string. We should only attempt
to set OAuth Config if thsoe values are non-empty.

Updates gitops-acl-action#33

Signed-off-by: Jenny Zhang <jz@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f