[Security Solution] API users can create rules with blank name and description

[MadameSheema]

Bug originally reported by: @rylnd

Describe the bug
API users can create rules with name and/or description fields of "" (empty string). While the UI forms prevent this, the backend allows it. While I haven't yet seen any errors (since the fields are still strings), it does cause some weird UI bugs:

When the rule renders in the Rules table, there is no name to display and thus no link to Details is clickable:

When detection alerts are generated, the Rule column is empty:

Alert fields display: name is -- while description is (Empty String):

Preconditions
Kibana 7.7+

To Reproduce
Steps to reproduce the behavior:

1. POST to the "create rule" endpoint (api/detection_engine/rules) with the following body:

{
  "description": "",
  "name": "",
  "severity": "low",
  "risk_score": 50,
  "type": "query",
  "enabled": false
}

Expected behavior
The request is rejected with a 4xx

On 2 Sep @spong commented:
Note, linking #65764 as the Rule column displayed as -- is the result of a separate issue.