Logs: disable invalid warning when no api_backend

Since API as a product, api_backend is no longer a required parameter of
the config. Due to the change, a warning message was in place on each
request due to invalid upstream.

With this change, in case of no api_backend, a nil object is returned
and no error message at all.

Fix THREESCALE-5225

Signed-off-by: Eloy Coto <eloy.coto@acalustra.com>

gateway/src/apicast/proxy.lua

@@ -172,6 +172,13 @@ function _M.get_upstream(service, context)
   if not service then
     return errors.service_not_found()
   end
+
+  -- Due to API as a product, the api_backend is no longer needed because this
+  -- can be handled by routing policy
+  if not service.api_backend then
+    return nil, nil
+  end
+
   local upstream, err = Upstream.new(service.api_backend)
   if not upstream then
     return nil, err

spec/proxy_spec.lua

@@ -57,6 +57,19 @@ describe('Proxy', function()
       assert.same(80, get_upstream({ api_backend = 'http://example.com' }):port())
       assert.same(8080, get_upstream({ api_backend = 'http://example.com:8080' }):port())
     end)
+
+    it("on invalid api_backend return error", function()
+      local upstream, err = get_upstream({ api_backend = 'test.com' })
+      assert.falsy(upstream)
+      assert.same(err, "invalid upstream")
+    end)
+
+    it("on no api_backend return nil and no error", function()
+      local upstream, err = get_upstream({})
+      assert.falsy(upstream)
+      assert.falsy(err)
+    end)
+
   end)
 
   describe('.authorize', function()