Only compare value if no key is provided

If you want to assert a redirect with only the session key it will fail.
Because we haven't given a value to compare it with.

```php
$this->assertRedirectedTo('signup/client', array('errors'));
```

`Failed asserting that null matches expected 'errors'.`

```php
    // $bindings = array('errors');
	public function assertSessionHasAll(array $bindings)
	{
		foreach ($bindings as $key => $value)
		{
            // $key = 0
            // $value = 'errors'
			$this->assertSessionHas($key, $value); // Fails
		}
	}
```