Modeler goes haywire if there are no models

[isovector]

If you don't add any mu mappings, you get a big chain of TH errors:

    • The type signature for ‘delEntity’ lacks an accompanying binding
        (The type signature must be given where ‘delEntity’ is declared)
    • In the Template Haskell quotation
        [d| delEntity :: Id -> System w -> System w
            getEntity :: Id -> System w -> Maybe (w)
            queryEntity :: Id -> Query w a -> System w -> Maybe a
            setEntity :: Id -> Setter w -> System w -> System w