Lol, no, not even close. First, what do you mean by 'an international arbitration court'? Such courts exist only because (and in the scope of) treaties where countries explicitly agree to be bound such courts, for example, signing a treaty with a clause stating 'disputes for this treaty shall be resolved in this way'. The only thing binding a sovereign is the treaties they "opt in". But Croatia has no treaties with Liberland, and Croatia has no obligations whatsoever to Liberland - the default state is that sovereign may do as it pleases, including 'abuse', war, slaughter, pillaging, etc - except when they, for example, sign a treaty confirming that there will be a border and they agree not to violate your side of border. If it's really 'terra nullius', then everybody, including Croatia, could do as they please, including burning the place to the ground and killing all the inhabitants, and the only treaty that comes to mind that would prevent that is Croatia's obligations with respect to the European Convention of Human Rights (but again, only because Croatia chose to take on these obligations) that would be violated by slaughter.

This is why a key precondition of being a state is recognition and border agreements with your neighbours - without that, the hypothetical state literally has no rights, not even the right to exist.