[FEATURE] Allow cx to keep no rules

[kaituo]

Is your feature request related to a problem?
if users don’t want any rule, they cannot delete default rule now.

What solution would you like?
Right now the default is added in AnomalyDetector's constructor. We can move it to Create/Update restful api. During Create, if user passes null or empty array in rules, add default rules. During update, don't add back if the rules are empty/null.

[krisfreedain]

Catch All Triage - 1