CPMLicense Should Silently Ignore Packags with empty source directories

[Jacobfaib]

The generated target ends up executing:

$ cmake \
  -DPRINT_license_FILE_NAME=/path/to/licenses.txt \ 
  -DPRINT_license_PACKAGES="...;CURL;..." \
  ... \
  -DCURL_SOURCE_DIR=""

And emits a warning:

WARNING: no license files found for package "CURL" in  .

IMO, the target should simply not emit a warning in this cases. CURL has come from a pre-existing system installation, so it won't have a source dir, and therefore no LICENSE to emit?