Error 500 when editing a Wiki page that does not exist

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

https://sentry.gitlab.net/gitlab/gitlabcom/issues/561605

NoMethodError: undefined method `content' for nil:NilClass
Did you mean?  concern
  projects/wikis_controller.rb:153:in `block in valid_encoding?'
    @page.content.encoding == Encoding::UTF_8
  gitlab/utils/strong_memoize.rb:26:in `strong_memoize'
    instance_variable_set(ivar(name), yield)
  projects/wikis_controller.rb:152:in `valid_encoding?'
    strong_memoize(:valid_encoding) do
  active_support/callbacks.rb:432:in `block in make_lambda'
    lambda { |target, _, &blk| target.send filter, &blk }
  active_support/callbacks.rb:417:in `block in invert_lambda'
    lambda { |*args, &blk| !l.call(*args, &blk) }
...
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NoMethodError: undefined method `content' for nil:NilClass
Did you mean?  concern

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.