How to assert for notNullish? for not empty string?

1) How to assert for notNullish?

`data != null`

I mean, I understand it can be done as:
```ts
t.NotNull(data)
t.NotUndefined(data)
```

But that is too much typing. Two lines of code for what can be done in a single `if`.

2) How to assert for not empty string?

`str != null && str != ''`

I cannot find it among the available functions.

Start by reading the existing NotNull and NotUndefined assertions and reviewing the library's available assertion functions. Determine how the requested not-nullish and non-empty-string assertions should fit the public API; done means both cases can be asserted with concise, typesafe calls.

Written by the indexing model from the issue text.