[Bugfix] Fix validating empty field list for fields query param

Closes laravel-json-api#225

CHANGELOG.md

@@ -3,6 +3,13 @@
 All notable changes to this project will be documented in this file. This project adheres to
 [Semantic Versioning](http://semver.org/) and [this changelog format](http://keepachangelog.com/).
 
+## Unreleased
+
+### Fixed
+
+- [#225](https://github.com/laravel-json-api/laravel/issues/225) Fix validation of empty field list for a resource type
+  in the `fields` query parameter, e.g. `/api/v1/employees?fields[employees]=`.
+
 ## [2.5.1] - 2023-01-23
 
 ### Fixed

composer.json

@@ -30,7 +30,7 @@
         "laravel-json-api/encoder-neomerx": "^2.0.1",
         "laravel-json-api/exceptions": "^1.1.1",
         "laravel-json-api/spec": "^1.2",
-        "laravel-json-api/validation": "^2.1.1",
+        "laravel-json-api/validation": "^2.1.2",
         "laravel/framework": "^8.76|^9.0"
     },
     "require-dev": {

tests/dummy/tests/Api/V1/Posts/ReadTest.php

@@ -180,6 +180,35 @@ public function testSparseFieldSets(): void
         $response->assertFetchedOneExact($expected)->assertIncluded([$author]);
     }
 
+    /**
+     * @return void
+     * @see https://github.com/laravel-json-api/laravel/issues/225
+     */
+    public function testSparseFieldSetsHasEmptyFieldList(): void
+    {
+        $post = Post::factory()->create();
+
+        $expected = $this->serializer
+            ->post($post)
+            ->only('author', 'slug', 'synopsis', 'title')
+            ->replace('author', ['type' => 'users', 'id' => $post->author]);
+
+        $author = $this->serializer
+            ->user($post->author)
+            ->withoutAttributes()
+            ->withoutRelationships();
+
+        $response = $this
+            ->withoutExceptionHandling()
+            ->jsonApi('posts')
+            ->sparseFields('posts', ['author', 'slug', 'synopsis', 'title'])
+            ->sparseFields('users', [])
+            ->includePaths('author')
+            ->get(url('/api/v1/posts', $expected['id']));
+
+        $response->assertFetchedOneExact($expected)->assertIncluded([$author]);
+    }
+
     public function testWithCount(): void
     {
         $post = Post::factory()