Composite rule for filled value only

This is not a good use case for using a composite rule. Composite rules are best used when a member in one or more other dimensions drives the mapping of the current dimension.

Here's how I would approach this:

1. Add code in your connector rule to detect null or empty string in the fields where blanks may come in, and replace them with something like <BLANK>
   1. You could use a form of logical expression.  I prefer using connector rules because they have more structure and are easier to see, code, and troubleshoot.
2. Create a Mask rule
   1. Expression: *
   2. Target Value: *
3. Create a One-To-One rule to map <BLANK> to whatever target member (e.g. None) you want

The transformation rules will run the One-To-One first, and whatever is left will be mapped by the Mask, anything not <BLANK> in this case.