libnm: fix an assert in nm_client_get_capabilities()

The length argument is optional. That makes sense, because the returned
array is NUL-terminated.

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/1136

Fixes: 21b008d0ff ('libnm: add nm_client_get_capabilities() to expose server Capabilities')

(cherry picked from commit 7f1840bbb6)

src/libnm-client-impl/nm-client.c

@@ -6261,7 +6261,6 @@ nm_client_get_capabilities(NMClient *client, gsize *length)
     NMClientPrivate *priv;
 
     g_return_val_if_fail(NM_IS_CLIENT(client), NULL);
-    g_return_val_if_fail(length, NULL);
 
     priv = NM_CLIENT_GET_PRIVATE(client);