Fix replacing required :input with null on PHP 8.1 (#36622)

Fixes an issue where data lacking one or more of the fields under
validation will cause tests to fail with an `ErrorException`.

That exception is a PHP deprecation warning triggered by the call to
`str_replace()` inside `replaceInputPlaceholder()`, which assumes the
value returned from `getDisplayableValue()` will always be a string.

src/Illuminate/Validation/Concerns/FormatsMessages.php

@@ -336,7 +336,7 @@ public function getDisplayableValue($attribute, $value)
             return $value ? 'true' : 'false';
         }
 
-        return $value;
+        return (string) $value;
     }
 
     /**