BUG: Refresh interval failed to work when there is only feature flag refresh is configured

[zhiyuanliang-ms]

A bug reported by our customer #527

Code

_nextCollectionRefreshTime will not be set if key value refresh is not configured (kvCollectionRefreshInterval will be null). This line will always set the current time as the next refresh due which will cause refresh is always due.

The result is that if user only enabled the feature flag refresh and set a refresh interval for feature flag, each time calling TryRefreshAsync, the refresh operation will always send requests. But the refresh will happen every time.

[amerjusupovic]

@jimmyca15 helped investigate offline, this does not manifest into an actual bug to users because of how the list of key-values and flags that need refresh are set on these lines. The provider normally requests app config for each watcher as it iterates over the list, but because the list is empty no additional calls are made. This logic still needs an update though, and a PR with the changes will be up soon.