Fix a bug introduced at a commit 306d30710305a52c1f5944b222530abaadb8aa49 - Even if a box instance is empty, finishContainer must be called. Otherwise, the page background is not painted.
Fix a bug introduced at a commit 306d30710305a52c1f5944b222530abaadb8aa49
- Even if a box instance is empty, finishContainer must be called. Otherwise, the page background is not painted.

vivliostyle/vivliostyle.js,vivliostyle/vivliostyle.js,kuad9/vivliostyle.js_study,nulltask/vivliostyle.js,vivliostyle/vivliostyle.js,nulltask/vivliostyle.js,kuad9/vivliostyle.js_study,nulltask/vivliostyle.js,nulltask/vivliostyle.js,kuad9/vivliostyle.js_study,vivliostyle/vivliostyle.js,kuad9/vivliostyle.js_study