fix(linter/explicit-module-boundary-types): debug assertion fail with top level return (oxc-project#12820)

we parse with:

https://github.com/oxc-project/oxc/blob/45206ddcda15bb3e41af5ad3d6e1174f35fe1df0/crates/oxc_linter/src/service/runtime.rs#L945

so it is possible the fns stack is empty

fixes oxc-project#12817

Author: camc314

crates/oxc_linter/src/rules/typescript/explicit_module_boundary_types.rs

@@ -476,9 +476,6 @@ impl<'a> Visit<'a> for ExplicitTypesChecker<'a, '_> {
             AstKind::ArrowFunctionExpression(arrow) => self.fns.push(Fn::Arrow(arrow)),
             AstKind::Class(_) => self.fns.push(Fn::None),
             AstKind::ReturnStatement(ret) => {
-                // returns outside of functions are semantic errors
-                let src: &str = self.ctx.source_text();
-                debug_assert!(!self.fns.is_empty(), "{src}");
                 let Some(f) = self.fns.last() else {
                     return;
                 };
@@ -1488,6 +1485,7 @@ mod test {
                 "function ErrorTrackingRules(): JSX.Element { return (<BindLogic><DndContext onDragEnd={({ active, over }) => { /**/ }}></DndContext></BindLogic>); }; export default ErrorTrackingRules;",
                 None,
             ),
+            ("export namespace B{return}", None),
         ];
 
         let fail = vec![