iptables: Fix handling of non-existent chains

iptables: Fix handling of non-existent chains

Since 694612adf87 the "compatibility" check considers non-existent
chains as "incompatible". This broke some scripts which used calls
like `iptables -L CHAIN404` to test for chain existence and expect
"No chain/target/match by that name." in the output.

This patch changes the logic of `nft_is_table_compatible()` to
report non-existent chains as "compatible" which restores the old
behavior.

Fixes: 694612adf87 ("nft: Fix selective chain compatibility checks")
Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1648
Signed-off-by: Jacek Tomasiak <jtomasiak@arista.com>
Signed-off-by: Jacek Tomasiak <jacek.tomasiak@gmail.com>
Signed-off-by: Phil Sutter <phil@nwl.cc>