One alternative way to find tags with no tag wiki is to visit the NEW tab on the tags pagetags page as indicated by @Bart in the comments.
This will give you the latest added tags and those often doesn't come yet with a wiki or excerpt.
As you see how many questions already have the tag you can judge how valuable an excerpt/wiki would be. As a bonus you spot misspelled tags (retag those questions so the misspelled tag gets removed at 03:00 UTC) or tags that either should be synonyms of existing tags or simply be replaced by a better tag.