list_modify removes elements when replacement is NULL (aka zaps)

Given that zap() is there it doesn't seem reasonable to allow NULLs to remove elements from the list:

```R
> list_modify(list(a = 1, b = 3), b = NULL)
$a
[1] 1

> list_modify(list(a = 1, b = 3), b = zap())
$a
[1] 1
```

It doesn't seem to be a way to insert NULS into a list with list modify currently. 