fix: prevent generatePath error when author is invalid in AuthorLabel

Wrap learnerPostsLink creation in useMemo with guard to prevent
'Missing :learnerUsername param' error. The generatePath function
was being called unconditionally during render even when the link
wouldn't be displayed, causing errors when author was null, undefined,
or the 'anonymous' string.

The fix ensures generatePath is only called when showUserNameAsLink
is true, which validates that author is a valid username.

Author: blarghmatey

src/discussions/common/AuthorLabel.jsx

@@ -101,17 +101,22 @@ const AuthorLabel = ({
     </>
   ), [author, authorLabelMessage, authorToolTip, icon, isRetiredUser, postCreatedAt, showTextPrimary, alert]);
 
-  const learnerPostsLink = (
-    <Link
-      data-testid="learner-posts-link"
-      id="learner-posts-link"
-      to={generatePath(Routes.LEARNERS.POSTS, { learnerUsername: author, courseId })}
-      className="text-decoration-none text-reset"
-      style={{ width: 'fit-content' }}
-    >
-      {!alert && authorName}
-    </Link>
-  );
+  const learnerPostsLink = useMemo(() => {
+    if (!showUserNameAsLink) {
+      return null;
+    }
+    return (
+      <Link
+        data-testid="learner-posts-link"
+        id="learner-posts-link"
+        to={generatePath(Routes.LEARNERS.POSTS, { learnerUsername: author, courseId })}
+        className="text-decoration-none text-reset"
+        style={{ width: 'fit-content' }}
+      >
+        {!alert && authorName}
+      </Link>
+    );
+  }, [showUserNameAsLink, author, courseId, alert, authorName]);
 
   return showUserNameAsLink
     ? (