FS-11360 Fix FS degradation over time in DTLS layer (especially if outdoing packets rate higher that incoming)

When we handle DTLS layer in do_dtls() and there are no incoming packets yet - we still call BIO_write() with NULL buffer and it is an error and BIO_write() will stuck for a long time in libssl/libcrypto error subsystem.
(that also possibly relates to fact that libssl/libcrypto init/deinit/error-handling is broken in general in FS, however that is another big issue)