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)
