Fix avoid undefined services field in getServices response

When the backend has no traces, the getServices endpoint returned
a nil services field, which is serialized as null in JSON.

This caused the UI to fail validation, since it expects an array.

This change ensures the services field is always returned as an
empty array.

Fixes #7921.

Signed-off-by: Sujal Shah <sujalshah28092004@gmail.com>

Author: sujalshah-bit

cmd/jaeger/internal/extension/jaegerquery/internal/apiv3/http_gateway.go

@@ -272,6 +272,9 @@ func (h *HTTPGateway) getServices(w http.ResponseWriter, r *http.Request) {
 	if h.tryHandleError(w, err, http.StatusInternalServerError) {
 		return
 	}
+	if len(services) < 1 {
+		services = []string{}
+	}
 	h.marshalResponse(&api_v3.GetServicesResponse{
 		Services: services,
 	}, w)