"Select Subscriptions..." shows in Azure Resources view if adding an empty item in "Azure Resource Groups: Selected Subscriptions" setting

[v-ruizh]

OS: All
Build Version: 0.9.7
Regression: Not a regression

Repro Steps:

1. Sign in with Azure account.
2. Reset user setting "Azure Resource Groups: Selected Subscriptions".
   
3. Refresh the Azure Resources view and all subscriptions show in Azure Resources view.
4. Add an invalid item in "Azure Resource Groups: Selected Subscriptions" setting, such as an empty item.
   
5. Refresh the Azure Resources view.
6. Check whether all subscriptions show in Azure Resources view.

Expect:
All subscriptions show in Azure Resources view.

Actual:
"Select Subscriptions..." shows in Azure Resources view.

More Info:
If both valid items and invalid items show in "Azure Resource Groups: Selected Subscriptions" setting, refresh the Azure Resources view, the corresponding subscriptions which added by valid items will show in Azure Resources view.