Handle missing description fields in create detector API

[ohltyler]

As pointed out in this issue, the create API does not reject the missing description field, even though it's listed as Required in the API. So, if a user submits an API request with all valid fields but a missing description field, then it leads to a missing description field in the system index, and parsing failures on the frontend and backend plugins, as it's expecting that field to exist (regardless of if it's value is populated or not). This creates confusion, since the raw response shows as successful when it actually is not. Details on the errors in the linked issue.

I see 2 possible paths forward:

1. Change to Not required, and handle the missing field in the backend similar to the other not required fields (e.g., filter_query/window_delay/category_field). Since it's listed as optional on the frontend, this aligns better from a user POV.
2. Leave as Required, and reject the request if it's missing in the body.