[Wiki] `@page` can be nil during `#edit`

Summary

In WikisController#valid_encoding?, we access page.content.encoding, but do not perform a nil check. This causes errors, such as:

https://sentry.gitlab.net/gitlab/gitlabcom/issues/1460527

Steps to reproduce

See https://sentry.gitlab.net/gitlab/gitlabcom/issues/1460527 - some edit operations are loading pages with nil content.

Output of checks

This bug happens on GitLab.com

Possible fixes

Guard @page.content access.