allow optimisticData to return undefined

[samsch]

the function passed as optimisticData in mutate gets two parameters, where currentData can explicitly be undefined regardless of Data (most recent updates from this PR: https://github.com/vercel/swr/pull/2708/files#diff-947adbc8aa4f84c3191ad83c84d1344a1fd311224f73a9f92e7b9eaa73860508R324). if you are using the function version of optimisticData though, it's very possible you can't create the whole object expected, so can we explicitly allow the return value of optimisticData to be undefined?

Right now this gets a type error because optimisticData infers that the return value should be DataType.

mutate('/somekey', undefined, {
	optimisticData: (previous?: DataType) => {
		if (previous == null) {
			return undefined;
		}
		return {
			...previous,
			updatedProperty: newValue,
		};
	},
});

It can be worked around by changing the first line to explicitly have the data type or undefined

mutate<DataType | undefined>('/somekey', undefined, {

The addition of an explicit type or undefined lets this work. but the value of swr.data is already allowed to be undefined.

Can we have optimisticData's inferred type also explicitly allow returning undefined?

[dmitriiprusakov]

@samsch Same question!