Another fix for #4727

This pull request https://github.com/zendframework/zf2/pull/4734 give problem with null values, and with InputFilter\Input::getValue()

example:


    public function testGetNotScalarValue()
    {
        $filter = $this->getMockBuilder('Zend\Filter\FilterInterface')
            ->disableOriginalConstructor()
            ->getMock();

        $filter->expects($this->any())
            ->method('filter')
            ->will($this->throwException(new \Zend\Filter\Exception\InvalidArgumentException()));
        //Filters such as: StripTags, StringToUpper, StringToLower, HtmlEntities throw exception if value is not scalar

        $filterChain = new \Zend\Filter\FilterChain();
        $filterChain->attach($filter);

        $input = new Input();
        $input->setFilterChain($filterChain);

        $value = null; //or any not scalar value array(), object, closure

        $input->setValue($value);

        $this->assertSame($value, $input->getValue()); //fail: give exception
        $this->assertFalse($input->isValid()); //also give exception

    }