PatternJsonProvider: provide a "#asNullIfEmpty{}" operation

[brenuart]

It would be very handy to have a #asNullIfEmpty{...} operation to convert an empty string into a null value.

People may want to use it together with the #property{} conversion specifier to return null instead of an empty string when the property is not defined. For instance, assuming the "foo" property is not defined, the following configuration:

<pattern>
   { "foo": "#asNullIfEmpty{%property{foo}}" }
</pattern>

... would produce a result like this:

{
   "foo": null
}