Fix .type for function

Summary: Do not return `None` from `.type` when type name is not known.

Reviewed By: JakobDegen

Differential Revision: D63708443

fbshipit-source-id: ed8f15f732c8eaf58aebbd56a2667a484f0cbcb5

Author: stepancheg

starlark/src/values/types/function.rs

@@ -171,7 +171,7 @@ impl<'v> StarlarkValue<'v> for NativeFunction {
     }
 
     fn get_attr(&self, attribute: &str, heap: &'v Heap) -> Option<Value<'v>> {
-        if let Some(s) = self.as_type.as_ref().map(|t| t.as_name()) {
+        if let Some(s) = self.as_type.as_ref().and_then(|t| t.as_name()) {
             if attribute == "type" {
                 return Some(heap.alloc(s));
             }