tdf#130595: Parameter query with :parameter IS NULL doesn't work

Problematic SQL here:
SELECT * FROM "Table1" WHERE "Name" = :name OR :name IS NULL

Firebird considers there are 2 parameters.

When providing an empty string, no pb, OPreparedStatement::setNull is called which is ok for both parameters

When providing a non empty string, we go to OPreparedStatement::setString. For first param no pb
but for second param, sql type is SQL_NULL so it must be taken into account
See https://www.firebirdsql.org/file/documentation/html/en/refdocs/fblangref25/firebird-25-language-reference.html#fblangref25-datatypes-special-sqlnull
for full details

Change-Id: I80dff259d85957e8547c098e4c48b642cce26804
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/127593
Reviewed-by: Lionel Mamane <lionel@mamane.lu>
Tested-by: Jenkins

connectivity/source/drivers/firebird/PreparedStatement.cxx

@@ -262,6 +262,12 @@ void SAL_CALL OPreparedStatement::setString(sal_Int32 nParameterIndex,
         setBoolean(nParameterIndex, boolValue);
         break;
     }
+    case SQL_NULL:
+    {
+        // See https://www.firebirdsql.org/file/documentation/html/en/refdocs/fblangref25/firebird-25-language-reference.html#fblangref25-datatypes-special-sqlnull
+        pVar->sqldata = nullptr;
+        break;
+    }
     default:
         ::dbtools::throwSQLException(
             "Incorrect type for setString",