doc/templates: fix page panic for not-found and version fallbacks
Created by: bobheadxi
Version fallbacks from customers are panicking again. It seems a new .Content
check that didn't check for non-nil was added, this PR fixes the check to avoid panics on fallbacks.
Similar fix from before for reference: https://github.com/sourcegraph/sourcegraph/pull/29557
Test plan
Fixed above, live below: