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browse/templates/top-navigation: Prevent VariableDoesNotExist exception

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browse/templates/top-navigation: Prevent VariableDoesNotExist exception

It exist cases where the vault_cooking template variable is set to None
(when browsing a snapshot branch targeting a content for instance), so
ensure to prevent VariableDoesNotExist exception thrown by django in
debug mode as it generates noise in debug output.