Fixed GFM autocomplete not loading on wiki pages

Closes #19580

CHANGELOG

@@ -14,6 +14,7 @@ v 8.10.0 (unreleased)
   - Let Workhorse serve format-patch diffs
   - Make images fit to the size of the viewport !4810
   - Fix check for New Branch button on Issue page !4630 (winniehell)
+  - Fix GFM autocomplete not working on wiki pages
   - Fix MR-auto-close text added to description. !4836
   - Fix issue, preventing users w/o push access to sort tags !5105 (redetection)
   - Add Spring EmojiOne updates.

app/views/layouts/_init_auto_complete.html.haml

@@ -1,7 +1,7 @@
 - project = @target_project || @project
+- noteable_class = @noteable.nil? ? nil : @noteable.class
 
-- if @noteable
-  :javascript
-    GitLab.GfmAutoComplete.dataSource = "#{autocomplete_sources_namespace_project_path(project.namespace, project, type: @noteable.class, type_id: params[:id])}"
-    GitLab.GfmAutoComplete.cachedData = undefined;
-    GitLab.GfmAutoComplete.setup();
+:javascript
+  GitLab.GfmAutoComplete.dataSource = "#{autocomplete_sources_namespace_project_path(project.namespace, project, type: noteable_class, type_id: params[:id])}"
+  GitLab.GfmAutoComplete.cachedData = undefined;
+  GitLab.GfmAutoComplete.setup();