cmd/pkgserver: handle unversioned value

This omits the field for an unversioned artifact, and only does so once on startup.

Signed-off-by: Ophestra <cat@gensokyo.uk>

cmd/pkgserver/index.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"errors"
 	"slices"
 	"strings"
 	"unique"
@@ -53,6 +54,12 @@ func (index *packageIndex) populate(cache *pkg.Cache, report *rosa.Report) (err
 			Metadata: rosa.GetMetadata(p),
 			Version:  rosa.Std.Version(p),
 		}
+		if m.Version == "" {
+			return errors.New("invalid version from " + m.Name)
+		}
+		if m.Version == rosa.Unversioned {
+			m.Version = ""
+		}
 
 		if cache != nil && report != nil {
 			m.ident = cache.Ident(rosa.Std.Load(p))