duration.humanize(): returns "a month" for `NaN`

**Describe the bug**

```console
> dayjs.duration(NaN).humanize()
"a month"
```

**Expected behavior**

```console
> dayjs.duration(NaN).humanize()
"invalid duration"
```

**Information**
- Day.js Version: 1.11.19
- OS: macOS 26.3
- Runtime: deno 2.6.9
- Time zone: GMT-06:00 CST (Central Standard Time)

Reproduce the reported dayjs.duration(NaN).humanize() result and trace the humanize() implementation in the Day.js source. Confirm that the invalid input returns "invalid duration" and add or update coverage for this case if the relevant test location is found.

Written by the indexing model from the issue text.