fix(babel-parser): chain off optionally chained keys named cla… (#11198)

* fix(babel-parser): chain off optionally chained keys named class and function

Fixes #11197

* Extract override as variable to satisfy typechecker

Flow did not trust that curContext.override would be defined

packages/babel-parser/src/tokenizer/index.js

@@ -233,8 +233,9 @@ export default class Tokenizer extends LocationParser {
       return;
     }
 
-    if (curContext.override) {
-      curContext.override(this);
+    const override = curContext?.override;
+    if (override) {
+      override(this);
     } else {
       this.getTokenFromCode(this.input.codePointAt(this.state.pos));
     }