Fix JSON printer handling of inf and nan

[jachris]

The JSON printer previously printed invalid JSON for inf and nan, which means that serve could sometimes emit invalid JSON, which is not handled well by platform/app. Instead, we now emit null.

- {"inf": inf.0, "nan": nan, "subnormal": 2.2250738585072e-310, "zero": -0.0}
+ {"inf": null, "nan": null, "subnormal": 2.2250738585072e-310, "zero": -0.0}

[mavam]

You may want to add a changelog entry for this bugfix.