obs: meaningful error if debian/changelog is empty

If there's an empty debian/changelog, packaging needs to be fixed in the
git repository of the project. Print a meaningful error instead of:

    File "/obs/lib/srcpkg.py", line 68, in get_version_for_feed
      if ":" in ret:
  TypeError: argument of type 'NoneType' is not iterable

Change-Id: I294d844ccccfa12599b6ba1def087e4a32d289e7

scripts/obs/lib/debian.py

@@ -73,14 +73,15 @@ def get_last_version_from_changelog(project):
     repo_path = lib.git.get_repo_path(project)
     changelog_path = f"{repo_path}/debian/changelog"
 
-    if not os.path.exists(changelog_path):
-        return None
+    assert os.path.exists(changelog_path), f"{project}: missing debian/changelog"
 
     changelog = open(changelog_path).read()
-    if not changelog:
-        return None
+    assert changelog, f"{project}: debian/changelog is empty"
 
-    return changelog.split("(", 1)[1].split(")", 1)[0]
+    ret = changelog.split("(", 1)[1].split(")", 1)[0]
+    assert ret, f"{project}: couldn't find last version in debian/changelog"
+
+    return ret
 
 
 def changelog_add_entry_if_needed(project, feed, version):

scripts/obs/lib/srcpkg.py

@@ -70,9 +70,6 @@ def get_version_for_feed(project, feed, conflict_version):
         if ":" in ret:
             ret = ret.split(":")[1]
 
-        if not ret:
-            ret = "0.0.0"
-
     # Append the conflict_version to increase the version even if the commit
     # did not change (OS#5135)
     if conflict_version: