This stops the checkvalidation (checkvalid / checkinvalid) target in
the makefile, which globs for `*(in)valid.cfg.yaml` files, from
calling `linkml validate` when no files match the pattern.
Previously, if there were no *.invalid.cfg.yaml files, we still made
it into the `@for ex in $</*invalid.cfg.yaml` loop (same for `*valid`)
-- but with the glob expression verbatim, producing errors like this:
"Invalid value for '--config': File
'src/flat-data/unreleased/validation/*.invalid.cfg.yaml' does not
exist."
It seems that make's wildcard function avoids the pitfall, while
correctly expanding the globs when files are present.
This is an alternative to a solution presented in #90, which excluded
checkinvalid from checkvalidation.
https://www.gnu.org/software/make/manual/html_node/Wildcard-Function.html
