I've been programming for over a quarter century, and I feel pretty confident that I've never written anything that could confuse 'null' with NULL. I can't even think of a language that would let you easily do this.
If web forms aren't accepting NULL, then somebody probably specifically programmed the word 'null' into a filter of disallowed entries. Probably to stop clerks from entering the word 'null' to mean empty string. This has nothing to do with null being a reserved word in many languages, I'll bet the forms that aren't accepting 'null' aren't accepting 'none' or 'empty' either.
If web forms aren't accepting NULL, then somebody probably specifically programmed the word 'null' into a filter of disallowed entries. Probably to stop clerks from entering the word 'null' to mean empty string. This has nothing to do with null being a reserved word in many languages, I'll bet the forms that aren't accepting 'null' aren't accepting 'none' or 'empty' either.