LibJS: Let isNaN() with no argument return true, add tests

Author: https://github.com/linusg
Commit: 8ff2881b1a
Pull-request: https://github.com/SerenityOS/serenity/pull/1923

Libraries/LibJS/Runtime/GlobalObject.cpp

@@ -137,9 +137,8 @@ Value GlobalObject::gc(Interpreter& interpreter)
 
 Value GlobalObject::is_nan(Interpreter& interpreter)
 {
-    if (interpreter.argument_count() < 1)
-        return js_undefined();
-    return Value(interpreter.argument(0).to_number().is_nan());
+    auto value = interpreter.argument(0).to_number();
+    return Value(value.is_nan());
 }
 
 }