Avoid unnecessary 500 panic when a commit doesn't exist (#28719) (#28721)

Avoid unnecessary 500 panic when a commit doesn't exist (#28719) (#28721)

Backport #28719 by wxiaoguang

In #26851, it assumed that `Commit` always exists when
`PageIsDiff==true`.

But for a 404 page, the `Commit` doesn't exist, so the following code
would cause panic because nil value can't be passed as string parameter
to `IsMultilineCommitMessage(string)` (or the StringUtils.Cut in later
PRs)

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>