build,json: fix generation with empty profiles

If the image generation doesn't add any profiles to the output the
*profile merge* will fail. To avoid that set an empty profile as
fallback.

Signed-off-by: Paul Spooren <mail@aparcar.org>

scripts/json_overview_image_info.py

@@ -42,7 +42,7 @@ def get_initial_output(image_info):
         output["profiles"][device_id]["images"].extend(profile["images"])
 
 # make image lists unique by name, keep last/latest
-for device_id, profile in output["profiles"].items():
+for device_id, profile in output.get("profiles", {}).items():
     profile["images"] = list({e["name"]: e for e in profile["images"]}.values())