Help with Specify the fields value with PHP code

I have a rule which combines two cck fields (field_part1 & field_part2) into a new field (field_combine) and it works great. My problem however is that although field_part2 always has a value field_part1 does not and I only want field combine created when both 1 & 2 have a value.

If I am reading the documentation correctly then I should be able to do this in the "Advanced: Specify the fields value with PHP code" and it will overwrite what I have in the token section ([node:field_part1-raw][node:field_part2-formated]).

So is this correct and if so how do I code it so that if field_part1 is null then field_combine is blank.

P.S. I can not do this by adding a condition.

Thank you.