make fraction part in DigitsConstraint optional

[lgemela]

raised based on discussion here graphql-java/graphql-java#3346

[bbakerman]

The tests are failing. Not sure why - please have a look

[lgemela]

should be fixed now. Optional does not really play well with error handling, I had to workaround it a bit.

Another solution would be to do this

 protected int getIntArg(GraphQLAppliedDirective directive, String argName) {
        return getIntArgOpt(directive, argName)
            .<RuntimeException>orElseThrow(() -> assertExpectedArgType(argName, "Int"));
    }

but IntelliJ does not like it that much, marking explicit type as unnecessary, which would be confusing to people.

[bbakerman]

Thanks for this