BTW: Instance cap explicitly CAN be set to null:

public FormValidation doCheckInstanceCapStr(@QueryParameter String value) {
            if (value == null || value.trim() == "") return FormValidation.ok();
            try {
                int val = Integer.parseInt(value);
                if (val > 0) return FormValidation.ok();
            } catch ( NumberFormatException nfe ) {}
            return FormValidation.error("InstanceCap must be a non-negative integer (or null)");
        }