net/tls: tls_is_tx_ready() checked list_entry

tls_is_tx_ready() checks that list_first_entry() does not return NULL.
This condition can never happen. For empty lists, list_first_entry()
returns the list_entry() of the head, which is a type confusion.
Use list_first_entry_or_null() which returns NULL in case of empty
lists.

Fixes: a42055e8d2c3 ("net/tls: Add support for async encryption of records for performance")
Signed-off-by: Pietro Borrello <borrello@diag.uniroma1.it>
Link: https://lore.kernel.org/r/20230128-list-entry-null-check-tls-v1-1-525bbfe6f0d0@diag.uniroma1.it
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
