Undefined index: input_filter_specs

As reported in [this thread](https://groups.google.com/a/zend.com/d/msgid/apigility-users/10e4e018-b4af-441b-836f-5e34701aefb4%40zend.com?utm_medium=email&utm_source=footer), a notice can pop up indicating that an index, `input_filter_specs`, does not exist on line 89 of `ZF\Apigility\Admin\Model\InputFilterModel`.

Solution: check for existence of that key before checking if a key exists in that array.