MetaStream type should allow factory to return nullish value

[earthlyreason]

According to the docstring for metaStream:

If the function returns null/undefined, no further action will be taken

So shouldn't the factory type here

umbrella/packages/rstream/src/metastream.ts

Line 108 in fe6cee7

factory: Fn<A, Subscription<B, B>>;

allow nullish values:

    factory: Fn<A, Subscription<B, B> | null | undefined>;

Currently, usages like this

    metaStream((x: string | undefined) => x && reactive(x));

get an error like this

Argument of type '(x: string | undefined) => "" | rs.Stream<string> | undefined' is not assignable to parameter of type 'Fn<string | undefined, Subscription<string, string>>'.
  Type '"" | Stream<string> | undefined' is not assignable to type 'Subscription<string, string>'.
    Type 'undefined' is not assignable to type 'Subscription<string, string>'. [2345]

[postspectacular]

You're absolutely right, @gavinpc-mindgrub! Have updated the sigs and also switched to the more general ISubscription interface as return type (even though currently everything is pretty much a subclass of Subscription only)...

[earthlyreason]

Many thanks @postspectacular!