map2 into a single map.
map1: A MAP expression.
map2: A MAP expression of the same key type as
func: A lambda function taking three parameters. The first parameter is the key, followed by the values from each map.
A MAP where the key matches the key type of the input maps and the value is typed by the return type of the lambda function.
If a key is not matched by one side the respective value provided to the lambda function is NULL.