nexthop: Remove redundant group len check in nexthop_create_group().

The number of NHA_GROUP entries is guaranteed to be non-zero in
nh_check_attr_group().

Let's remove the redundant check in nexthop_create_group().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20250319230743.65267-6-kuniyu@amazon.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>