Empty characters show up in annual blast occasionally (5)

I believe the answer is:

nulls

'empty characters' is the definition.
(meaningless letters in cryptography)

'annual blast occasionally' is the wordplay.
'occasionally' means one should take alternating letters.
'annual'+'blast'='annualblast'
The alternate letters of 'annualblast' are 'NULLS'.

'show up in' is the link.

I've seen this clue in The Independent.

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