Fix na_if Translation for Oracle

[carvalhomb]

Translate na_if to NULLIF rather than NULL_IF when using an Oracle connection.

Reference: https://docs.oracle.com/en/database/oracle/oracle-database/18/sqlrf/NULLIF.html

(Related to #32)

[hadley]

I think it actually should be NULLIF everywhere:

• Oracle: https://docs.oracle.com/en/database/oracle/oracle-database/18/sqlrf/NULLIF.html
• MySQL: https://dev.mysql.com/doc/refman/8.0/en/control-flow-functions.html#function_nullif
• SQLite: https://www.sqlite.org/lang_corefunc.html#nullif
• Postgres: https://www.postgresql.org/docs/8.4/functions-conditional.html
• MS SQL: https://docs.microsoft.com/en-us/sql/t-sql/language-elements/nullif-transact-sql?view=sql-server-2017