Learn Croatian: Ova ladica je prazna, unutra ništa ne vidim. - This drawer is empty, I see nothing inside.

This is because of mandatory double negation in Croatian! Whenever you use a negative pronoun like ništa (nothing), the verb must also be negative. You literally have to say 'I don't see nothing' (ništa ne vidim).

Is ništa in the nominative or accusative case here?

It is in the accusative case because it is the direct object of the verb vidjeti (to see). However, for the word ništa, the nominative and accusative forms look exactly the same, so it doesn't change its shape here.

Why does the adjective end in 'a' (prazna)?

Adjectives have to match the gender of the noun they describe. Since ladica (drawer) is a feminine noun, the base masculine adjective prazan (empty) drops its extra 'a' and takes the feminine ending -a to become prazna.

Can I change the word order and say ne vidim ništa unutra?

Yes, you absolutely can. Croatian word order is quite flexible. Putting unutra (inside) first just puts a little more emphasis on where you are looking, but the grammatical meaning stays exactly the same.