[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/ssh.go

@@ -14,6 +14,9 @@ func ParseSSHSpecs(sl []string) ([]*controllerapi.SSH, error) {
 	}
 
 	for _, s := range sl {
+		if s == "" {
+			continue
+		}
 		parts := strings.SplitN(s, "=", 2)
 		out := controllerapi.SSH{
 			ID: parts[0],