Types: findOne(Async): found document may be null

[dtkdtk]

If the document is not found, Cursor<null> will be returned

[dtkdtk]

Also, please fix types for removeAsync():

- removeAsync(query: any, options: Nedb.RemoveOptions): Promise<number>;
+ removeAsync(query: any, options?: Nedb.RemoveOptions): Promise<number>;
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