content/static: fix null reference errors in unit.js
content/static: fix null reference errors in unit.js

Fixes golang/go#43434

Change-Id: I0af7549c9acdb16202d6d77ca2993eea6cb35f7f
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/293841
Trust: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite