fix text fields and exists operator

src/Schema/Ui/Text.php

@@ -34,7 +34,7 @@ class Text implements FieldInterface, RequiredInterface
 
     public function format(array $input, array $output): array
     {
-        $value = (string)$input[$this->name] ?? null;
+        $value = !empty($input[$this->name]) ? (string)$input[$this->name] : null;
         $output[$this->name] = (new Nullable($this->nullable, $value, ""))->value();
         return $output;
     }