Add check for empty request params before splitting

Fixes the issue with sorting and filtering of a datagrid for `oro_multiple_entity `. 

It's related to #194 and to [this request](https://github.com/orocrm/crm/issues/91#issuecomment-65087911)

@DimaSoroka the user @alsma-magecore recommended I should do this and as you can see I did. 

Let me know if I should do anything else related to this.

Thanks