def IF_NULL(expression, _else):
return {'<STR_LIT>': [expression, _else]}<EOL>
Evaluates an expression and returns the value of the expression if the expression evaluates to a non-null value.
See https://docs.mongodb.com/manual/reference/operator/aggregation/ifNull/
for more details
:param expression: expression that will be evaluated
:param _else: value or expression if 'expression' is null
:return:  Aggregation operator