InfluxDB 2(api/orgs): Filter by UserID with invalid ID returns current organization instead of an error

__Steps to reproduce:__
```
>curl -H "Authorization: Token my-token" http://localhost:9999/api/v2/orgs?userID=aaaaaaaaaaaaaaaa
{
"links": {
"self": "/api/v2/orgs"
},
"orgs": []
}
>curl -H "Authorization: Token my-token" http://localhost:9999/api/v2/orgs?userID=aaaaaa
{
"links": {
"self": "/api/v2/orgs"
},
"orgs": [
{
"links": {
"buckets": "/api/v2/buckets?org=my-org",
"dashboards": "/api/v2/dashboards?org=my-org",
"labels": "/api/v2/orgs/061039e69dd07000/labels",
"logs": "/api/v2/orgs/061039e69dd07000/logs",
"members": "/api/v2/orgs/061039e69dd07000/members",
"owners": "/api/v2/orgs/061039e69dd07000/owners",
"secrets": "/api/v2/orgs/061039e69dd07000/secrets",
"self": "/api/v2/orgs/061039e69dd07000",
"tasks": "/api/v2/tasks?org=my-org"
},
"id": "061039e69dd07000",
"name": "my-org",
"description": "",
"createdAt": "2020-07-27T15:35:35.287608197Z",
"updatedAt": "2020-07-27T15:35:35.287608297Z"
}
]
}
```

__Expected behavior:__
GET request with invalid userID to /api/v2/orgs returns an error about invalid ID.

__Actual behavior:__
GET request with valid, but not existing, userID to /api/v2/orgs returns empty list.
GET request with invalid UserID to /api/v2/orgs returns current organization.

__Environment info:__

* InfluxDB version: Beta 15

Start at the GET /api/v2/orgs endpoint and reproduce the two userID cases described in the issue. Trace how the endpoint parses the userID and handles an invalid ID versus a valid but nonexistent ID. Done means invalid IDs return an error, while valid nonexistent IDs still return an empty list.

Written by the indexing model from the issue text.