How to define a data type in a Custom Field formula?

How to define a data type in a Custom Field formula?

Hello, I want to create a Custom Field with a formula that returns me a value, if there is a value in that field, if not it should return Blank. The problem is that the field I am calling is a Multiselect Pick List, so is not a regular string field. 

What I am trying to do is to use the If funtion with the Contains function, however I haven´t found the way to write it just by stating that if there is something there return this value, if not return nothing... 

Could you help me to clarify this? is there a way to do it? or should I write all the possibilities of the Multiselect pick list on the formula? Thanks.