printers CHANGE try to print all values

Even unresolved unions. The printed value
will be wrong only in extremely rare cases.

Fixes sysrepo/sysrepo#1069
diff --git a/src/printer_json.c b/src/printer_json.c
index 4b13918..5b726a2 100644
--- a/src/printer_json.c
+++ b/src/printer_json.c
@@ -173,6 +173,7 @@
     case LY_TYPE_INST:
     case LY_TYPE_INT64:
     case LY_TYPE_UINT64:
+    case LY_TYPE_UNION:
     case LY_TYPE_DEC64:
         json_print_string(out, leaf->value_str);
         break;