ZF2-523: InputFilter does not validate empty values
It is impossible to make a validator for case when value can be empty depending on other values.
For example, if some checkbox is checked, then empty value is allowed. If checkbox is not checked, empty value is not allowed.
checkbox name: doesNotHaveOwner (I understand that using negative is bad, but our case needs this behaviour) input name: ownerName
if doesNotHaveOwner == 0, then ownerName is mandatory (As both are empty, then no validator is invoked) if doesNotHaveOwner == 1, then ownerName can be empty.