[v0.19] bake: remove empty values set by --set

These parser functions are called for `--set` to
resolve entitlement paths that would be automatically added
and will fail for empty value.

The empty values would already be ignored but in v0.19 is
done after merging the `--set` values and then calling
parse again.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

util/buildflags/cache.go

@@ -14,6 +14,9 @@ import (
 func ParseCacheEntry(in []string) ([]*controllerapi.CacheOptionsEntry, error) {
 	outs := make([]*controllerapi.CacheOptionsEntry, 0, len(in))
 	for _, in := range in {
+		if in == "" {
+			continue
+		}
 		fields, err := csvvalue.Fields(in, nil)
 		if err != nil {
 			return nil, err