Fix map.patch for null items

Two questions, then:

1. Just so I understand it, if we leave `Key->Anything` and the entry being searched has the same `Key` type but a different `Item` type, we will have to execute the code up to the `item==entryitem` comparison (that will return `false`, of course). Is the performance better even in that case? Or are you assuming that most of the times we won't have that exact case, and then it's an acceptable trade-off?

2. Would it be even better using `Object->Anything`, so we avoid the reification of `Key`, too?