Incorrect error response when creating a project

When creating a project without a Content-Type header, I get the following error:

```{
"@context": "https://bluebrain.github.io/nexus/contexts/error.json",
"@type": "MissingQueryParam",
"reason": "Request is missing required query parameter 'rev'."
}```

Start by reproducing project creation without a Content-Type header and inspect how the request is routed to error handling. Trace why the response reports a missing 'rev' query parameter, then verify that the API returns the correct error for this request.

Written by the indexing model from the issue text.