Overwriting a value with an undefined/null field will not change the old value

**Describe the bug**

When overwriting an object's value with undefined, it will leave the old value intact

```bash
❯ yq -n '.foo = "abc" | .foo = .bar'
```

**Actual behavior**

```yaml
foo: abc
```

**Expected behavior**

```yaml
foo: null
```

**Additional context**

```bash
❯ jq -n '.foo = "abc" | .foo = .bar'
{
  "foo": null
}

❯ yq -n '.foo = "abc" | .foo = null'
foo: null
```