The `callHierarchy` function should warn the user when
`textDocument/prepareCallHierarchy` didn't resolve an entity and
return, rather than calling the `callHierarchy/{incoming,outgoing}Calls`
method with an empty object - which is encoded as an empty list (which
doesn't respect language server specification for the
`callHierarchy/incomingCalls` call).
01691c5447
