UI: Don't save/overwrite browser docks if CEF hasn't loaded

OBS only populates the extraBrowserDocks list if `cef` is valid. By
saving docks regardless of whether `cef` is valid, this can result in
the user's custom docks being cleared unexpectedly.
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f