The goal of Moralis is to always offer you fully resolved metadata so that you don't have to resolve it yourself and save load time in your app.
Why is it null?
Some NFTs have their metadata hosted on centralized servers. These servers sometimes have rate limits preventing Moralis from fully indexing the collections that have their metadata stored on such servers. In such cases the metadata may not be resolved. We are working all the time to extend our coverage of metadata.
How to get metadata?
You can use the Resync Metadata endpoint to ReSync the metadata of an NFT, and for an NFT collection, you can use the Sync NFT contract endpoint to sync the NFT contract metadata.