Revert to default spec dirs if none are specified

Instead of accepting an empty list of spec dirs, we explicitly
revert to the default spec dirs to ensure that we don't
configure a cache with no spec dirs specified.

Signed-off-by: Evan Lezar <elezar@nvidia.com>

Author: elezar

pkg/cdi/spec-dirs.go

@@ -43,6 +43,10 @@ var (
 
 // WithSpecDirs returns an option to override the CDI Spec directories.
 func WithSpecDirs(dirs ...string) Option {
+	// If no spec dirs are specified use the default spec dirs.
+	if len(dirs) == 0 {
+		return WithSpecDirs(DefaultSpecDirs...)
+	}
 	return func(c *Cache) {
 		specDirs := make([]string, len(dirs))
 		for i, dir := range dirs {