Fix TemplateFileServer writing empty messages resulting in zero length DataBlocks being created and later passed to SocketIO::Send()

Found while testing m_ssl_gnutls, fix suggested by @Adam-
Turn BinarySocket::Write() into a no-op when called with l == 0
Merge pull request #54 from attilamolnar/2.0+webcpanelfix

Don't create zero length DataBlocks in BinarySocket::Write()