Remove the ExternRef::null constructor

Shorter and clearer to just use `None`.

crates/wasmtime/src/ref.rs

@@ -26,11 +26,6 @@ impl ExternRef {
         ExternRef { inner, store }
     }
 
-    /// Creates a `Null` reference.
-    pub fn null() -> Option<Self> {
-        None
-    }
-
     /// Get this reference's store.
     ///
     /// Returns `None` if this reference outlived its store.

crates/wasmtime/src/values.rs

@@ -63,7 +63,7 @@ macro_rules! accessors {
 impl Val {
     /// Returns a null `externref` value.
     pub fn null() -> Val {
-        Val::ExternRef(ExternRef::null())
+        Val::ExternRef(None)
     }
 
     /// Returns the corresponding [`ValType`] for this `Val`.