declareQueries should not accept queries returning undefined (closes #54)

[giogonzo]

tests performed

declaring a query that resolves to undefined yields:

      TS2345: Argument of type '{ a: QueryReturn<IOTSDictToType<{}> & IOTSDictToType<{}> & IOTSDictToType<{}>,...' is not assignable to parameter of type 'Queries'.
  Property 'a' is incompatible with index signature.
    Type 'QueryReturn<IOTSDictToType<{}> & IOTSDictToType<{}> & IOTSDictToType<{}>, T[] | undefined>' is not assignable to type 'QueryReturn<any, mixed>'.
      Type 'T[] | undefined' is not assignable to type 'mixed'.
        Type 'undefined' is not assignable to type 'mixed'.

[nemobot]