sql_mode="oracle" - add Oracle function NVL

Oracle has a regularly used function called NVL(expr1,expr2). It appears to be the same as mariadb expression IFNULL(expr1,expr2). This request is to create a NVL function for Oracle compatibility.

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Syntax
The syntax for the NVL function in Oracle/PLSQL is:

NVL( string1, replace_with )
Parameters or Arguments
string1
The string to test for a null value.
replace_with
The value returned if string1 is null.
Returns
The NVL function returns a substitute value.

This task also included function NVL2(expr1,expr2,expr3)
If expr1 is TRUE .. then it returns exp2, otherwise returns expr3