[racket] Value Formatting ~r Routine and Non-Rational +nan.0

Currently  the `~r' format routine, which accepts rationals, barfs on
+nan.0 and siblings, which are not rationals.  My current use case is
simply displaying sub-ranges of vectors of flonums.  It is not a problem to
check for +nan.0 and explicitly display it. I was just wanted to toss it
out as a don't know what is the "right thing" for the `~r' format routine
to do here.

> (define: x : Flonum +nan.0)
> (define: y : Flonum 3.145962)
> (~r y)
- : String
"3.145962"
> (~r x)
~r: contract violation
 expected: rational?
 given: +nan.0
...
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