[Custom handler] Remove "Outputs not set on http response" warnings

These are printed when the outputs or return value don't exist:

```
Outputs not set on http response for invocationId...
ReturnValue not set on http response for invocationId...
```

Not sure if it's necessary to warn on them. Perhaps these can be `Information` or omitted.

Also, if the response contains an empty object for `Outputs`, it still prints the warning. Sometimes a function really doesn't have an output or return value.

```
[2020-09-30T04:44:39.066] {"Outputs":{},"Logs":["hello from queue trigger","\"\\\"{\\\\n \\\\\\\"id\\\\\\\": 42\\\\n}\\\"\""],"ReturnValue":""}
[2020-09-30T04:44:39.067] Received http response for httpTrigger function: 'queueTrigger' invocationId: '08f2780f-042a-477d-8786-ef666f3fd171'
[2020-09-30T04:44:39.068] Outputs not set on http response for invocationId:08f2780f-042a-477d-8786-ef666f3fd171
```

Not marking as a GA blocker but would be good to address this. @pragnagopa @yojagad