Is there a single word meaning 'not null'?

It is based on your preference I think, isset / is / set / exist / has / ...

But don't you think code readability is more important to understand and write, instead of shortening every word

In PHP there is a function named isset - I think there's no shorter version than this.

VALUED

The caveat here is NULL may be considered a value in some languages, so universal use of VALUED as the antithesis of NULL is not appropriate.

In PHP however, VALUED cleanly opposes NULL.