fix return types for enums (#57)

* Add MixedValueToScalarConverter for fixing a problem with SQL binding of non-scalar values such as enums

* fix return types

---------

Co-authored-by: korotkov <korotkov@mubert.com>

src/Converters/MixedValueToScalarConverter.php

@@ -8,16 +8,16 @@
 
 class MixedValueToScalarConverter
 {
-    public function handle(mixed $value): int|float|bool|string
+    public function handle(mixed $value): int|float|bool|string|null
     {
         if (is_scalar($value) || $value === null) {
             return $value;
         } elseif (is_object($value) && PHP_VERSION_ID >= 80100 && enum_exists(get_class($value))) {
             return $value->value;
         } elseif (is_object($value) && method_exists($value, '__toString')) {
             $value->__toString();
-        } else {
-            throw new BulkValueTypeIsNotSupported($value);
         }
+
+        throw new BulkValueTypeIsNotSupported($value);
     }
 }