[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/secrets.go

@@ -11,6 +11,9 @@ import (
 func ParseSecretSpecs(sl []string) ([]*controllerapi.Secret, error) {
 	fs := make([]*controllerapi.Secret, 0, len(sl))
 	for _, v := range sl {
+		if v == "" {
+			continue
+		}
 		s, err := parseSecret(v)
 		if err != nil {
 			return nil, err