Add tests for GraphProperty::isNull()

Author: clue

tests/Property/PropertyGraphTest.php

@@ -11,6 +11,7 @@ public function testEmptyIsEdgeless()
 
         $alg = new GraphProperty($graph);
 
+        $this->assertTrue($alg->isNull());
         $this->assertTrue($alg->isEdgeless());
         $this->assertFalse($alg->isTrivial());
     }
@@ -22,6 +23,8 @@ public function testSingleVertexIsTrivial()
 
         $alg = new GraphProperty($graph);
 
+        $this->assertFalse($alg->isNull());
+        $this->assertTrue($alg->isEdgeless());
         $this->assertTrue($alg->isTrivial());
     }
 }