fix(FeildtypeOptions): fix error for empty values when it's a single option

Author: dadish

src/Type/Fieldtype/FieldtypeOptions.php

@@ -9,9 +9,10 @@
 use ProcessWire\GraphQL\Utils;
 use ProcessWire\GraphQL\Type\Fieldtype\Traits\SetValueTrait;
 use ProcessWire\GraphQL\Type\Fieldtype\Traits\FieldTrait;
+use ProcessWire\Page;
 
 class FieldtypeOptions
-{ 
+{
   use FieldTrait;
   use SetValueTrait;
 
@@ -63,6 +64,34 @@ public static function type($field)
     return $type;
   }
 
+  public static function field($field)
+  {
+    return Cache::field($field->name, function () use ($field) {
+      // description
+      $desc = $field->description;
+      if (!$desc) {
+        $desc = "Field with the type of {$field->type}";
+      }
+
+      return [
+        'name' => $field->name,
+        'description' => $desc,
+        'type' => self::type($field),
+        'resolve' => function (Page $page, array $args) use ($field) {
+          $fieldName = $field->name;
+
+          // If a single option field does not have an id, then it means it's empty.
+          if (!self::isMultiple($field) && !$page->$fieldName->id) {
+            return null;
+          }
+
+          return $page->$fieldName;
+        }
+      ];
+    });
+  }
+
+
   public static function inputType($field)
   {
     return Cache::type(self::getName($field), function () use ($field) {

test/Field/Page/Fieldtype/FieldtypeOptions/CaseFiveTest.php

@@ -0,0 +1,37 @@
+<?php
+
+/**
+ * Empty options field should return null.
+ */
+
+namespace ProcessWire\GraphQL\Test\FieldtypeOptions;
+
+use ProcessWire\GraphQL\Utils;
+use ProcessWire\GraphQL\Test\GraphQLTestCase;
+
+class CaseFiveTest extends GraphQLTestCase
+{
+  const settings = [
+    "login" => "admin",
+    "legalTemplates" => ["city"],
+    "legalFields" => ["options_single"],
+  ];
+
+  public function testValue()
+  {
+    $city = Utils::pages()->get("template=city, options_single=''");
+    $query = "{
+      city (s: \"id=$city->id\") {
+        list {
+          options_single {
+            title
+            value
+            id
+          }
+        }
+      }
+    }";
+    $res = self::execute($query);
+    self::assertNull($res->data->city->list[0]->options_single, "Empty option field should return null.");
+  }
+}