blob: threat 0-sized entry as non-existient on append with EXTHDR

This is follow-up fix for 886117b:
    "blob: fix append of non-exisient entries with EXTHDR"

This part is responsible for code path where entry is actually exists
but has zero size (i.e. prepared entry), therefore must be threated as
non-existent one.

Reported by: kiwi@