It is an impersonal sentence. As for the Genitive, consider the word "нечего" a separate pronoun with a meaning "there is nothing to ..." Since Genitive is used with negatives, the pronoun doesn't really have a Nominative form. It has other forms, though, and it does divide into не + чего when you have a preposition (i.e. when used with verbs that require a certain preposition for a specific meaning, like "думать о ..."). It is also used in the meaning "Shouldn't", "No reason to".

Examples:
Мне не с кем играть.
Незачем так волноваться.
Им нечем писать.
Мне нечем похвастаться.
Нечему радоваться.