chore: clean up check_* in dbi-driver

[detule]

There are a number of arguments ( encoding, timezone, timeout, etc ) for which allow_null is set to TRUE on one hand, and on the other we simply pass them through the C/++ layer ( where they are strongly typed ).

• Verify that in fact they are not allowed to be NULL ( expect Rcpp to complain ).
• Update the arguments to the check_* methods; probably better to throw a nicely formatted error.

[detule]

Currently:

conn <- dbConnect(odbc::odbc(), dsn = "mssql_oem_db", encoding = NULL )
Error: Expecting a single string value: [type=NULL; extent=0].
> conn <- dbConnect(odbc::odbc(), dsn = "mssql_oem_db", timezone = NULL )
Error: Expecting a single string value: [type=NULL; extent=0].
> conn <- dbConnect(odbc::odbc(), dsn = "mssql_oem_db", timezone_out = NULL )
Error: Expecting a single string value: [type=NULL; extent=0].
> conn <- dbConnect(odbc::odbc(), dsn = "mssql_oem_db", timeout = NULL )
Error in if (is.infinite(timeout)) { : argument is of length zero