Check if project is not null before processing

What does this MR do and why?

Check if the project is not null before processing.

In cases where the project is returned null by GraphQL, the start function throws an error. This MR fixes that.

Screenshots or screen recordings

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How to set up and validate locally

TBA

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #412877 (closed)

app/assets/javascripts/code_review/signals.js

@@ -21,6 +21,11 @@ async function observeMergeRequestFinishingPreparation({ apollo, signaler }) {
       query: getMr,
       variables: { projectPath, iid },
     });
+
+    if (!currentStatus.data.project) {
+      return;
+    }
+
     const { id: gqlMrId, preparedAt } = currentStatus.data.project.mergeRequest;
     let preparationObservable;
     let preparationSubscriber;