1.10.5 has weird type bug on duration.add()

```ts
import dayjs from 'dayjs'
import duration, { Duration } from 'dayjs/plugin/duration'
dayjs.extend(duration)

type MyType = { duration?: Duration }
const obj: MyType = {}
dayjs.duration(10, 'second').add(obj.duration ?? 0) // fail

const duration: Duration | undefined = undefined
dayjs.duration(10, 'second').add(duration ?? 0) // pass
```

play ground link: https://www.typescriptlang.org/play?#code/JYWwDg9gTgLgBAEwIYE8BWBnOAzKERwDky6GhAUKJLIgK5RIzAQB2ANHAN5wAi9jzFnAC+OPAWKpMAejAAbWgHNgLaQn5NWFEpgB0AUwAeMfSwQAKdQ00sAlOXIwUYfXACyKACrPXAXi501oIA-ABcvBqCIuQAxqwY8BAARmjhHt4ucP6cwuQ6GLpWAqzmAIwADByEGPpxZoS2ukgIFslohZGscMHBcOW2cNLSOEjAcg51CYHFLOF8QV0APnC0ZvrYKvoIWStrGyxbeVIFRTZllUQ1dQgNTS2WnUI9fQNDcGBIGBjkQA

dayjs.duration(10, 'second').add(obj.duration ?? 0) will throw compile error:
```
No overload matches this call.
The last overload gave the following error.
Argument of type 'Duration | 0' is not assignable to parameter of type 'Duration'.
Type 'number' is not assignable to type 'Duration'.(2769)
```

I suspect this some weird typescript type infer error

however, can you just allow duration.add(undefined) to pass compiler, which equiv as duration.add(0)

Start with the TypeScript declarations for the dayjs/plugin/duration entry point and reproduce the reported calls from the issue. Check how add() handles Duration, numbers, and undefined, then verify that the requested call type-checks without changing the runtime behavior.

Written by the indexing model from the issue text.