Add support for lower PHP version

Problem/Motivation

This module gives white screen of death during installation. The reason is - we are not using variables in empty() in some places. From empty() documentation:

Prior to PHP 5.5, empty() only supports variables; anything else will result in a parse error. In other words, the following will not work: empty(trim($name)). Instead, use trim($name) == false.

Places where we are not using variables inside empty():
- if (!empty($field_handler->element_classes())) {}
- if (!empty($field_handler->element_wrapper_classes())) {}

Proposed resolution

Use variable instead of direct value from function.

Remaining tasks

N/A

User interface changes

N/A

API changes

N/A

Data model changes

N/A