use simple decimal query to detect decimal char

use simple decimal query to detect decimal char

Fixed issue where the ``nls_session_parameters`` view queried on first
connect in order to get the default decimal point character may not be
available depending on Oracle connection modes, and would therefore raise
an error.  The approach to detecting decimal char has been simplified to
test a decimal value directly, instead of reading system views, which
works on any backend / driver.

Fixes: #8744
Change-Id: I39825131c13513798863197d0c180dd5a18b32dc