Fallback to false when remove_source_branch is null.

app/assets/javascripts/vue_merge_request_widget/stores/mr_widget_store.js

@@ -51,7 +51,7 @@ export default class MergeRequestStore {
     this.cancelAutoMergePath = data.cancel_merge_when_pipeline_succeeds_path;
     this.removeWIPPath = data.remove_wip_path;
     this.sourceBranchRemoved = !data.source_branch_exists;
-    this.shouldRemoveSourceBranch = data.remove_source_branch;
+    this.shouldRemoveSourceBranch = data.remove_source_branch || false;
     this.onlyAllowMergeIfPipelineSucceeds = data.only_allow_merge_if_pipeline_succeeds || false;
     this.mergeWhenPipelineSucceeds = data.merge_when_pipeline_succeeds || false;
     this.mergePath = data.merge_path;