fix: fix thin bar problem (#8350)

[kanitw]

• https://github.com/vega/vega-lite/issues/8350 Fix thin bar problem

See the added bar_group_thin example before and after the behavior fix commit.

(Other Vega diffs do not produce image diffs, so this shouldn't introduce any regression.)

📦 Published PR as canary version: 5.4.1--canary.8349.f5dfbbd.0

:sparkles: Test out this PR locally via:

npm install [email protected]
# or 
yarn add [email protected]

[vega-org-bot]

:rocket: PR was released in v5.5.0 :rocket: