Make tests more strict.

assertEmpty() will also pass on 0, false, null and '' which is not what
we want. Matching exactly to '' is more what we're looking for.

tests/TestCase/Network/Session/DatabaseSessionTest.php

@@ -130,7 +130,7 @@ class DatabaseSessionTest extends TestCase
         $this->assertEquals($expected, $result);
 
         $result = $this->storage->read('made up value');
-        $this->assertEmpty($result);
+        $this->assertSame('', $result);
     }
 
     /**
@@ -143,7 +143,7 @@ class DatabaseSessionTest extends TestCase
         $this->storage->write('foo', 'Some value');
 
         $this->assertTrue($this->storage->destroy('foo'), 'Destroy failed');
-        $this->assertEmpty($this->storage->read('foo'), 'Value still present.');
+        $this->assertSame('', $this->storage->read('foo'), 'Value still present.');
     }
 
     /**
@@ -161,7 +161,7 @@ class DatabaseSessionTest extends TestCase
 
         sleep(1);
         $storage->gc(0);
-        $this->assertEmpty($storage->read('foo'));
+        $this->assertSame('', $storage->read('foo'));
     }
 
     /**