ExportNamedDeclaration#source can be null (#23909)

e.g. this named declaration has a source:
```javascript
export {foo} from './bar.js';
```

and this one does not:
```javascript
export const foo = 10;
```

Try it out at https://astexplorer.net/ to confirm.

types/babel-types/index.d.ts

@@ -422,7 +422,7 @@ export interface ExportNamedDeclaration extends Node {
     type: "ExportNamedDeclaration";
     declaration: Declaration;
     specifiers: ExportSpecifier[];
-    source: StringLiteral;
+    source: StringLiteral | null;
 }
 
 export interface ExportSpecifier extends Node {