Don't create a ProxyText object if there's no text in the buffer.

The pathological case here is when using Dragon's 'delete that' command.  It will send a series of 'backspace' keys.  Without this change, every 'backspace' key press will be preceded by an empty ProxyText call, effectively doubling the amount of RPC calls we'd make.
Merge pull request #51 from nirvdrum/client_window_optimization

Don't create a ProxyText object if there's no text in the buffer.