Remove empty function on variable function

I'm getting "Fatal error: Can't use function return value in write context in app/core/gui.class.php on line 74"

```php

if ( !empty($bgp_module::$module_definition['module_options']) ) {

				if ( !empty($bgp_module::getModuleOption( 'empty_navbar' )) ) {

					$this->empty_navbar = boolval( $bgp_module::getModuleOption( 'empty_navbar' ) );
				}

				if ( !empty($bgp_module::getModuleOption( 'no_sidebar' )) ) {

					$this->no_sidebar = boolval( $bgp_module::getModuleOption( 'no_sidebar' ) );
				}
			}
```
This is most likely thrown from using empty on a variable function.

I have removed !empty and replaced it with != null.