Fix NRE on trying to update all when there's nothing to update

[HebaruSan]

Problem

1. Set up a repo with an invalid URL as your only repo
2. Close CKAN
3. Start CKAN
4. The failed downloads window will appear. Click abort.
5. Now you have an empty mod list
   (The above steps are just one specific way that I've found to get to this point. It may or may not be how other users might get here, but the below problem happens regardless of the specific path taken.)
6. Click Update all even though there are no mods to update
7. An NRE is thrown

Cause

ManageMods.mainModList.full_list_of_mod_rows is null when the grid is empty, and ManageMods.MarkAllUpdates accesses its .Values property.

Changes

• Now ManageMods.mainModList.full_list_of_mod_rows is set to an empty dictionary on initialization, so it can always be accessed safely, and clicking the button with an empty list will do nothing.
• A foreach loop in ManageMods.ChangeSetUpdated now uses a deconstructor to get the key and value separately instead of a kvp variable to represent the key value pair. This isn't needed to fix anything, but it's nice for readability.

Fixes #4053.