Treating empty strings as single space.
The other option is to treat them as null.
Those are only two options while using dbrpcparam.

It is kinda safe to put single space strings into db if len function is used to check for emtpy strings.
Len strips trailing spaces, so returns 0 for single space string.

Links:
  https://github.com/pymssql/pymssql/issues/243
   http://stackoverflow.com/questions/2025585/len-function-not-including-trailing-spaces-in-sql-server