[Bugfix] Allow resource id to be zero

Closes #445

Author: lindyhopchris

CHANGELOG.md

@@ -2,6 +2,12 @@
 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
+- [#445](https://github.com/cloudcreativity/laravel-json-api/issues/445)
+Allow resource identifier to be zero.
+
 ## [1.5.0] - 2019-10-14
 
 ### Added

src/Object/IdentifiableTrait.php

@@ -75,7 +75,7 @@ public function getId()
             throw new RuntimeException('Id member is not a string.');
         }
 
-        if (empty($id)) {
+        if (empty($id) && '0' !== $id) {
             throw new RuntimeException('Id member is an empty string.');
         }
 

tests/lib/Unit/Object/ResourceIdentifierTest.php

@@ -146,4 +146,10 @@ public function testInvalidId($id)
         $this->expectException(RuntimeException::class);
         ResourceIdentifier::create('posts', $id)->getId();
     }
+
+    public function testZeroId()
+    {
+        $identifier = ResourceIdentifier::create('posts', '0');
+        $this->assertSame('0', $identifier->getId());
+    }
 }