🧹 dinglehopper: Disable metrics in JSON report, too

qurator/dinglehopper/templates/report.json.j2

@@ -1,8 +1,10 @@
 {
     "gt": "{{ gt }}",
     "ocr": "{{ ocr }}",
+{% if metrics %}
     "cer": {{ cer|json_float }},
     "wer": {{ wer|json_float }},
+{% endif %}
     "n_characters": {{ n_characters }},
     "n_words": {{ n_words }}
 }