libnm: fix warning when setting wrong ethtool ternary value

$ nmcli connection modify dummy1 ethtool.feature-rx a
  (process:3077356): GLib-WARNING **: GError set over the top of a previous GError or uninitialized memory.
  This indicates a bug in someone's code. You must ensure an error is NULL before it's set.
  The overwriting error message was: 'a' is not valid; use 'on', 'off', or 'ignore'
  Error: failed to modify ethtool.feature-rx: 'a' is not valid; use [true, yes, on], [false, no, off] or [unknown].

Fixes: e5b46aa38a ('cli: use nmc_string_to_ternary() to parse ternary in _set_fcn_ethtool()')

src/libnmc-setting/nm-meta-setting-desc.c

@@ -4383,7 +4383,7 @@ _set_fcn_ethtool(ARGS_SET_FCN)
     if (!nmc_string_to_ternary_full(value,
                                     NMC_STRING_TO_TERNARY_FLAGS_IGNORE_FOR_DEFAULT,
                                     &t,
-                                    error)) {
+                                    NULL)) {
         g_set_error(error,
                     NM_UTILS_ERROR,
                     NM_UTILS_ERROR_INVALID_ARGUMENT,