contrib: have apiage fail if no source data is available

It is kind of pointless to let the tool run if no source data is
present, so treat empty/missing source as an error.

Signed-off-by: John Mulligan <jmulligan@redhat.com>

contrib/apiage.py

@@ -278,6 +278,12 @@ def main():
     api_src = read_json(cli.source) if cli.source else {}
     api_tracked = read_json(cli.current) if cli.current else {}
 
+    if not api_src:
+        print(
+            f"error: no source data found (path: {cli.source})", file=sys.stderr
+        )
+        sys.exit(1)
+
     if cli.current_tag:
         tag_to_versions(cli, cli.current_tag)