[compiler] Fix copyright script (#33003)

Don't try to open directories
---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/facebook/react/pull/33003).
* #33004
* __->__ #33003
* #33002

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Co-authored-by: Jordan Brown <jmbrown@meta.com>

compiler/scripts/copyright.js

@@ -51,6 +51,9 @@ if (hasErrors) {
 }
 
 function processFile(file) {
+  if (fs.lstatSync(file).isDirectory()) {
+    return;
+  }
   let source = fs.readFileSync(file, 'utf8');
 
   if (source.indexOf(META_COPYRIGHT_COMMENT_BLOCK) === 0) {