Support updating back to empty fields

[raul-te]

The ThousandEyes API misses updating back to empty fields. For example:

$ curl -d '{"contentRegex":"Needle"}' -H "Content-Type: application/json" -H "Accept: application/json" -H "Authorization: Bearer redacted" https://api.thousandeyes.com/v6/tests/http-server/<test_number>/update

$ curl -sd '{"contentRegex":""}' -H "Content-Type: application/json" -H "Accept: application/json" -H "Authorization: Bearer redacted" https://api.thousandeyes.com/v6/tests/http-server/<test_number>/update | jq | grep Needle
      "contentRegex": "Needle",

It happens in many fields, such as contentRegex, username, password, customHeaders, alertRules, etc.

This is a limitation. For example, it's not possible to disable alert rules entirely if an alert rule was already defined.

This issue has been reported internally and a fix is pending.