Update page mutation without specifying tags

[splumhoff]

If no tags are submitted e.g. via mutation{pages {update()}} the update will fail. This fixes the issue, same solution as in the pages create method:

https://github.com/requarks/wiki/blob/b5b4b0880ae26f4b137242267b6674b51af8688c/server/models/pages.js#L327

[NGPixel]

You're missing the scenario where you want to remove all tags. Your current code would skip saving because the array length is 0.