koji: Handle undeclared branches differently than empty set...

Which is semantically different, as explained in review but from user
perspective both will fail (but logs will be more accurate).

Also relocate the smaller scope closer to test, for readability.

Relate-to: #774 (comment)
Co-authored-by: Yann Dirson <yann.dirson@vates.tech>
Signed-off-by: Philippe Coval <philippe.coval@vates.tech>

Author: rzr

scripts/koji/koji_build.py

@@ -54,7 +54,9 @@ def check_commit_is_available_remotely(dirpath, hash, target, warn):
             raise Exception("Building with a user target requires using --pre-build or --test-build.\n")
         try:
             supported_branches = PROTECTED_TARGETS.get(target)
-            if supported_branches:
+            if supported_branches is None:
+                raise Exception(f"Protected target {target} has no allowed branches.")
+            else:
                 for branch in supported_branches:
                     try:
                         if is_remote_branch_commit(dirpath, hash, branch):
@@ -65,8 +67,6 @@ def check_commit_is_available_remotely(dirpath, hash, target, warn):
                 else:
                     raise Exception(f"The current commit is not the last commit of any remote branches {supported_branches}.\n"
                                     f"This is required when using the protected target {target}.")
-            else:
-                raise Exception(f"Protected target {target} has no allowed branches.")
         except Exception as e:
             if warn:
                 print(f"warning: {e}", flush=True)