Learn Russian: Около пустой аудитории никого нет. - There is no one near the empty classroom.

Even though -ой is a common masculine nominative ending, here пустой is the feminine genitive form of пустая (empty). We use the genitive case because the preposition около (near) requires it. Feminine adjectives in the genitive usually end in -ой or -ей.

Why does аудитории end in -ии instead of the usual feminine genitive endings?

The dictionary form is аудитория (classroom). For feminine nouns ending in -ия, the genitive singular ending is always -ии. This is a special spelling rule to remember, as it differs from the standard -ы or -и endings.

Why are there two negative words, никого and нет?

Russian grammar requires a double negative. Whenever you use a negative pronoun like никого (nobody), you must also make the sentence's verb or predicate negative. That is why нет (there is not) is required here.

Why is никого used instead of the dictionary form никто?

The word нет always forces the missing person or thing into the genitive case. Никого is the genitive form of никто (nobody).