feat: ignore variables with targetValue nil

[lukasjarosch]

The targetValue of variables only evaluates to nil if their actual value is undefined.
This case might occur if one defines an inventory value which uses bash / environment variables, which
ook like skipper variables, but actually aren't.

If in the inventory you have

myclass:
  test: |
    echo "${MY_ENV_VARIABLE}" 

Then ${MY_ENV_VARIABLE} would be treated as skipper variable, but without a targetValue, hence it would be
replaced with nil. This PR changes the default behaviour so that these variables are not replaced at all.