osbuild-image-info: exit with non-zero value on empty report

For the purpose of using this tool in tests (specifically for manifest
tests where we diff image-info reports), it is important that the tools
exists with non-zero value if the final report is empty.

Signed-off-by: Tomáš Hozza <thozza@redhat.com>

tools/osbuild-image-info

@@ -2950,6 +2950,10 @@ def main():
     else:
         report = analyse_image(target)
 
+    if not report:
+        print(f"Failed to analyse {target}: no information gathered in the report", file=sys.stderr)
+        sys.exit(1)
+
     json.dump(report, sys.stdout, sort_keys=True, indent=2)