Bugfix for interfaces and abstract classes

Bug:
"Empty Entities: Empty Method" violations for empty methods in interfaces and abstract classes.

Problem:
The count method return 0 also if the element does not exists.

Fix:
Check first if the node exists.

Test:
There is currently not tests in the project. So here is my test class.
interface TestInterface {
    public function test();
}

abstract class TestAbstract
{
    abstract public function test();
}

class Test
{
    public function test() { }
}
Merge pull request #1 from Wohlie/master

Bugfix for interfaces and abstract classes