libotutil: Allow no variant in ot_util_variant_builder_from_variant()

For programming convenience in cases where we may or may not have a
GVariant to initialize the builder with.

src/libotutil/ot-variant-utils.c

@@ -260,16 +260,20 @@ ot_util_variant_builder_from_variant (GVariant            *variant,
                                       const GVariantType  *type)
 {
   GVariantBuilder *builder = NULL;
-  gint i, n;
   
   builder = g_variant_builder_new (type);
   
-  n = g_variant_n_children (variant);
-  for (i = 0; i < n; i++)
+  if (variant != NULL)
     {
-      GVariant *child = g_variant_get_child_value (variant, i);
-      g_variant_builder_add_value (builder, child);
-      g_variant_unref (child);
+      gint i, n;
+
+      n = g_variant_n_children (variant);
+      for (i = 0; i < n; i++)
+        {
+          GVariant *child = g_variant_get_child_value (variant, i);
+          g_variant_builder_add_value (builder, child);
+          g_variant_unref (child);
+        }
     }
     
   return builder;