guard against next[id] as null

I was getting this error because sometimes next[id] is null, but typeof null === 'object':

```
/home/substack/projects/enliven/node_modules/patcher/index.js:155
        if(next[id].equals !== undefined && next[id].equals(prev[id])) {
                   ^
TypeError: Cannot read property 'equals' of null
```