OPT: check element fisrt (#2429)

run ` if (!element)` at first to avoid rest code running.

If` if (!element)` equals true, then stop executing the rest other wise we run `element.forEach(item => walkTree(item, context, visitor, newContext));`  all the time at first

src/getDataFromTree.ts

@@ -57,12 +57,12 @@ export function walkTree(
   ) => boolean | void,
   newContext: Map<any, any> = new Map(),
 ) {
-  if (Array.isArray(element)) {
-    element.forEach(item => walkTree(item, context, visitor, newContext));
+  if (!element) {
     return;
   }
-
-  if (!element) {
+
+  if (Array.isArray(element)) {
+    element.forEach(item => walkTree(item, context, visitor, newContext));
     return;
   }