Implement native function of  nvl(82)

**Description**

Convert Spark's built-in function nvl to execute as a native function.

**Spark function name:**
nvl

**Method description:**

nvl(expr1, expr2) - Returns expr2 if expr1 is null, or expr1 otherwise.



**Examples**:

> SELECT nvl(NULL, array('2'));
 ["2"]



**To Native function**

f::core::nvl()