Non-existent, the object unknown. A thing with no validity (7)

I believe the answer is:

nullity

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'a thing with no validity' is the definition.
I can't judge whether this definition defines the answer.

'non-existent the object unknown' is the wordplay.
'non-existent' becomes 'null' (I am not sure about this - if you are sure you should believe this answer much more).
'the object' becomes 'it' (I've seen this in other clues).
'unknown' becomes 'y' (algebra).
'null'+'it'+'y'='NULLITY'

Can you help me to learn more?

(Other definitions for nullity that I've seen before include "Nothingness" , "State of being void" , "Invalid act" , "being without force" .)

I've seen this clue in the Sydney Morning Herald.

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