AES-CBC pre-determined IV vs. null IV
There's no practical difference between zero IV and any other constant IV here.

With some older ciphers that have a small enough keyspace (or weaknesses that allow reducing it) you could have a rainbow table for the encryption of the zero vector which might make zero IV a weaker choice in some cases, but that would be impossible for AES with its 128-256 bit keys.

Note that when a key is only used to encrypt a single plaintext, a constant IV is secure.