Dispatch whitelist should clearly indicate when the default whitelist will be applied

Currently, the OOTB config for the dispatch whitelist is an empty value, which is interpreted as "use the internal default". Since an empty value can be interpreted to mean "there is no whitelist", this should be replaced with some special value (e.g., DEFAULT) to explicitly indicate that the internal default value will be applied. An empty value will then mean "there is no whitelist".

The comment for the configuration property should also clearly communicate this.