Do not pass nil to s-contains?

It is tolerated (and sometimes expected) for the exec-path list to contain a nil value, to represent the default directory.

With previous code, this resulted in s-contains? emitting the error "Wrong type argument: stringp, nil" -- this fix passes the default directory when it encounters a nil value in exec-path.
Merge pull request #4 from empiricalthought/master

Do not pass nil to s-contains?