net: sockets: tls: Ignore empty iovec entries in sendmsg

According to `sendmsg()` man pages, the `struct msghdr` can contain
empty records (iov_len equal to 0). Ignore them in TLS `sendmsg()`
implementation to avoid unnecessary calls to mbed TLS.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>