There is nothing to be seen, 當中(to be seen)意思是....

其實許多時不定詞的被動語態與主動語能是相通的，如：
There is no time to lose( or to be lost)
There is a lot to do yet (or to be done)
但to see和to be seen卻並不是同一意思，如：
There are many sights to see(=worth seeing) in England（在英國有許多值得遊覽的名勝
There are many sights to be seen（＝that can be seen) in England.
（在英國可以看見許多名勝）
所以，你的句子的意思是：無東西可看
希望幫到你！

100% correct
to be seen = passive voice of infinitive (to see), so the meaning is
被看