Fix isset warning

layouts/partials/posts/utteranc.html

@@ -1,9 +1,11 @@
-{{- if and (isset .Site.Params.utteranc "repo") (not (eq .Site.Params.utteranc.repo "" )) (eq (.Params.disable_comments | default false) false) -}}
-<script src="https://utteranc.es/client.js"
-  repo= "{{ .Site.Params.utteranc.repo }}"
-  issue-term="{{ default "title" .Site.Params.utteranc.issueTerm }}"
-  theme="{{ default "github-light" .Site.Params.utteranc.theme }}"
-  crossorigin="anonymous"
-  async>
-</script>
+{{- if isset .Site.Params "utteranc" -}}
+  {{- if and (isset .Site.Params.utteranc "repo") (not (eq .Site.Params.utteranc.repo "" )) (eq (.Params.disable_comments | default false) false) -}}
+  <script src="https://utteranc.es/client.js"
+    repo= "{{ .Site.Params.utteranc.repo }}"
+    issue-term="{{ default "title" .Site.Params.utteranc.issueTerm }}"
+    theme="{{ default "github-light" .Site.Params.utteranc.theme }}"
+    crossorigin="anonymous"
+    async>
+  </script>
+  {{- end -}}
 {{- end -}}