Adjusted assertions to match NullEngine::write() returned value

tests/TestCase/Cache/CacheTest.php

@@ -235,7 +235,7 @@ class CacheTest extends TestCase
         Cache::disable();
         $this->_configCache();
 
-        $this->assertNull(Cache::write('no_save', 'Noooo!', 'tests'));
+        $this->assertTrue(Cache::write('no_save', 'Noooo!', 'tests'));
         $this->assertFalse(Cache::read('no_save', 'tests'));
         $this->assertNull(Cache::delete('no_save', 'tests'));
     }
@@ -739,7 +739,7 @@ class CacheTest extends TestCase
 
         Cache::disable();
 
-        $this->assertNull(Cache::write('key_2', 'hello', 'test_cache_disable_1'));
+        $this->assertTrue(Cache::write('key_2', 'hello', 'test_cache_disable_1'));
         $this->assertFalse(Cache::read('key_2', 'test_cache_disable_1'));
 
         Cache::enable();
@@ -754,7 +754,7 @@ class CacheTest extends TestCase
             'path' => CACHE . 'tests'
         ]);
 
-        $this->assertNull(Cache::write('key_4', 'hello', 'test_cache_disable_2'));
+        $this->assertTrue(Cache::write('key_4', 'hello', 'test_cache_disable_2'));
         $this->assertFalse(Cache::read('key_4', 'test_cache_disable_2'));
 
         Cache::enable();
@@ -763,7 +763,7 @@ class CacheTest extends TestCase
         $this->assertSame(Cache::read('key_5', 'test_cache_disable_2'), 'hello');
 
         Cache::disable();
-        $this->assertNull(Cache::write('key_6', 'hello', 'test_cache_disable_2'));
+        $this->assertTrue(Cache::write('key_6', 'hello', 'test_cache_disable_2'));
         $this->assertFalse(Cache::read('key_6', 'test_cache_disable_2'));
 
         Cache::enable();