User-defined types - How to define default value null but also string type as allowed

[slavizh]

Is your feature request related to a problem? Please describe.
I would like to define a property that has default value of null but also to allow any string to be assigned. I was trying this:

type test = {
  foo: null | string
}

but it throws errors. If I define this:

type test = {
  foo: null
}

I cannot set a string value for foo.
If such feature is not available I would like to request it.

Describe the solution you'd like
See above.

[jeskew]

Closing as duplicate of #8973. There is an open PR for this feature (#9454), but it is blocked pending the rollout of a change in the ARM runtime and may not be included in the upcoming release.