avm1: Fix String.lastIndexOf(undefined)

core/src/avm1/globals/string.rs

@@ -319,7 +319,7 @@ fn last_index_of<'gc>(
         .encode_utf16()
         .collect::<Vec<u16>>();
     let pattern = match args.get(0) {
-        None | Some(Value::Undefined) => return Ok(Value::Undefined),
+        None => return Ok(Value::Undefined),
         Some(s) => s
             .clone()
             .coerce_to_string(activation)?

core/tests/swfs/avm1/string_methods/output.txt

@@ -140,6 +140,12 @@ undefined
 // s.lastIndexOf(null)
 17
 
+// s.lastIndexOf(undefined)
+-1
+
+// "hello undefined hi".lastIndexOf(undefined)
+6
+
 // slice
 // s.slice(1, 4)
 ell