Handle null input in JsonOps.convertTo

[Papierkorb2292]

The current implementation of JsonOps.convertTo throws a null pointer exception when input is null by calling input.getAsJsonPrimitive() without checking for null. Instead, the implementation only checks whether input is an implementation of JsonNull. This is problematic because some of the other methods in JsonOps replace a JsonNull instance with null when returning it, which makes it possible that input is null instead of a JsonNull instance. An example of such a method is JsonOps.getMapValues.

This is an issue with the implementation of convertTo and not just a problem with call sites elsewhere, because convertTo also uses getMapValues when converting maps and it recursively calls convertTo on the returned map values, which means an input like {"value": null} is also going to throw an error even though it is a valid JsonObject instance.

This pull request adds a null check besides the instanceof check for JsonNull, such that a null input value and a value of type JsonNull are handled the same in JsonOps.convertTo.

This also fixes #62