fix: defensively handle nil maps and slices in marshaling (#18418)

Adds a custom marshaler to handle some cases where nils were being
marshaled to nulls, causing the web UI to throw an error.

---------

Co-authored-by: Steven Masley <stevenmasley@gmail.com>

enterprise/coderd/idpsync.go

@@ -836,6 +836,9 @@ func (api *API) idpSyncClaimFieldValues(orgID uuid.UUID, rw http.ResponseWriter,
 		httpapi.InternalServerError(rw, err)
 		return
 	}
+	if fieldValues == nil {
+		fieldValues = []string{}
+	}
 
 	httpapi.Write(ctx, rw, http.StatusOK, fieldValues)
 }