Allow emptyFields for PRG component in order for isSearch to stay correct

[dereuromark]

This came up a few times before as an issue if I recall correctly.

In my case

	echo $this->Form->input('search');
	echo $this->Form->input('has_missing_translation', ['type' => 'checkbox']);

The 0 posted will always result in the URL ?has_missing_translation=0, which then also affects the isSearch and the reset button in the process.
The empty values should be seen as such (as the CakeDC one did). This is a non-invasive minimal BC safe addition that resolves the issue.

[dereuromark]

Is this OK as minor prior to the major bump? It is fully BC.
Or include it in the other two PRs?

[dereuromark]

Thank you!