mnl: don't send empty set elements netlink message to kernel

mnl: don't send empty set elements netlink message to kernel

The following command:

 # nft --debug=mnl add rule x y flow table xyz { ip saddr timeout 30s counter }

breaks with EINVAL. The following netlink message is causing the
problem:

...
----------------        ------------------
|  0000000044  |        | message length |
| 02572 | R--- |        |  type | flags  |
|  0000000004  |        | sequence number|
|  0000000000  |        |     port ID    |
----------------        ------------------
| 02 00 00 00  |        |  extra header  |
|00008|--|00002|        |len |flags| type|
| 78 79 7a 00  |        |      data      |       x y z
|00008|--|00004|        |len |flags| type|
| 00 00 00 01  |        |      data      |
|00006|--|00001|        |len |flags| type|
| 78 00 00 00  |        |      data      |       x
----------------        ------------------
...

This is incorrect since this describes no elements at all, so it is
useless.  Add upfront check before iterating over the list of set
elements so the netlink message is not placed in the batch.

This patch also adds a set so flow tables are minimally covered.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>