What is null key encryption?

You must understand the context. This is meant for IPSec, other parts of IPSec provide integrity (nobody can change the data that's in transit), and authentication (certainty about who sent the data), and an encryption scheme provides confidentiality (only the intended recipient can read the data).

In some situations, IPSec may be used for Integrity and Authentication, but not Confidentiality (some situations may prohibit encryption). You can then configure IPSec with a null key encryption mechanism. This way you still get two out of three benefits of IPSec: Nobody can mess with your message in-transit, and the communicating parties are certain about who they are talking to.

Of course, if Batman was trying to keep the data secret in the movie, it was probably just an obscure joke/easter egg aimed at people who know what the Null Key encryption is.

There is no Null Key Encryption. It is just plain fiction.

The RFC you linked is a... fun RFC? The Null Algorithm described there is indeed a very very powerful algorithm. It encrypts your plaintext

Hello World

To the ciphertext

Hello World

I heavily doubt this algorithm will ever be broken :-)