Don't save screenshots that don't have any useful data on them

Docker/annotate_header.py

@@ -105,6 +105,8 @@ with Image.open(output_file) as im:
                     anchor="rm",
                     font=font,
                 )
+    if len(found_needles) == 0:
+        exit(0)
     im = im.resize((im.width // 2, im.height // 2))
     im.save(output_file, "PNG")
     print(json.dumps({"new_file":