dbConnect: clean up argument checks

[detule]

Closes #848

• See issue for examples of how NULLs for specific arguments in dbConnect are handled prior to this patch.
• With this patch:

┃  > dbConnect(odbc::odbc(), dsn = "mssql_oem_db", encoding = NULL )
┃                                                  Error in `.local()`:
┃  ! `encoding` must be a single string, not `NULL`.
┃  Run `rlang::last_trace()` to see where the error occurred.
┃  > dbConnect(odbc::odbc(), dsn = "mssql_oem_db", timezone = NULL )
┃                                                  Error in `.local()`:
┃  ! `timezone` must be a single string, not `NULL`.
┃  Run `rlang::last_trace()` to see where the error occurred.
┃  > dbConnect(odbc::odbc(), dsn = "mssql_oem_db", timezone_out = NULL )
┃                                                  Error in `.local()`:
┃  ! `timezone_out` must be a single string, not `NULL`.
┃  Run `rlang::last_trace()` to see where the error occurred.
┃  > dbConnect(odbc::odbc(), dsn = "mssql_oem_db", timeout = NULL )
┃                                                  Error in `.local()`:
┃  ! `timeout` must be a number, not `NULL`.
┃  Run `rlang::last_trace()` to see where the error occurred.

Also disallowed NA for timeout, as it's not clear from the documentation how that would be handled ( we allow Inf to mean "no timeout". IMO that's intuitive enough. ) Also documented the usage of NA for the timeout argument. No change in prior behavior.

Thanks!

[simonpcouch]

Nice!