Dummy address causes Config validation error

Problem/Motivation

From UpdateEmailBuilder, notification email address for update.settings is overridden with dummy value.

But this way, it fails config validation with Configuration Inspector cause
dummy is not a valid email address.

Proposed resolution

We should use either valid email address or empty value for config_overrides section in UpdateEmailBuilder

 *   config_overrides = {
 *     "update.settings" = {
 *       "notification" = { "emails" = { "dummy" } },
 *     },
 *   },