Make return of findPathFromEnclosingNodeToTopLevel @nullable

It can clearly return `null`

Fixes #4903

COPYBARA_INTEGRATE_REVIEW=#4903 from msridhar:add-nullable-annot 33fe001
PiperOrigin-RevId: 737557860

Author: msridhar

check_api/src/main/java/com/google/errorprone/util/ASTHelpers.java

@@ -440,7 +440,8 @@ public static ExpressionTree stripParentheses(ExpressionTree tree) {
    * Given a TreePath, finds the first enclosing node of the given type and returns the path from
    * the enclosing node to the top-level {@code CompilationUnitTree}.
    */
-  public static <T> TreePath findPathFromEnclosingNodeToTopLevel(TreePath path, Class<T> klass) {
+  public static <T> @Nullable TreePath findPathFromEnclosingNodeToTopLevel(
+      TreePath path, Class<T> klass) {
     if (path != null) {
       do {
         path = path.getParentPath();