[BUG] diffDeep lacks the undefined argument checks of equalsDeep()

**Describe the bug**

in` equalsDeep`:
```
    // Fast comparisons
    if (!object1 || !object2) {
      return false;
    }
```

No such check in diffDeep, so we get to:

```
    for (const parm in object2) { //TODO: strings are iterable and this is a bug
      const value1 = object1[parm];
      const value2 = object2[parm];
```
and the second line blows up.

