build,json: fix build failure in case no data is found

Only collect arch_packages if actually generating any output.

Fixes: commit f09b9319 ("build,json: store arch_packages in profiles.json"(
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Paul Spooren <mail@aparcar.org>

scripts/json_overview_image_info.py

@@ -32,7 +32,7 @@ for json_file in work_dir.glob("*.json"):
                 image_info["profiles"][device_id]["images"][0]
             )
 
-
+if output:
     default_packages, output["arch_packages"] = run(
         [
             "make",
@@ -49,8 +49,6 @@ default_packages, output["arch_packages"] = run(
     ).stdout.splitlines()
 
     output["default_packages"] = default_packages.split()
-
-if output:
     output_path.write_text(json.dumps(output, sort_keys=True, separators=(",", ":")))
 else:
     print("JSON info file script could not find any JSON files for target")