How to check target filed value empty or not using messsage mapping?

Hello,

I have to send data from 3rd party--->PI---> ECC as of now we using bypass there is no mapping,now my requirement is EX:ten fileds data I am sending PI--->ECC here I have to check phone no field if phone is already there then I need to store the phone no into another field.

1.Need to check target field value is empty or not.

2.If value is there then no need to update,no need change.

3.Soure valuse will be store into some other filed into Target side.

How can I do this in Message Mapping.

Thanks

Kumar