Fixes the number of directives displayed associated with snapshot

[duranb]

Currently if you select a snapshot that didn't have any activities associated to it, the displayed number of directives in the plan snapshot bar will display the total directives that exist in the latest revision. This fix ensures that if no directives were present at the time of the snapshot creation, the displayed number of directives will display "0".

To test:

1. Create plan
2. Before adding directives, create a snapshot
3. After a snapshot is created, then add a few
4. Create another snapshot
5. Repeat steps 3-4 as many times as you want, but make sure you mentally keep track of how many directives exist during the creation of a snapshot
6. Open the plan metadata panel and select the very first snapshot
7. Verify that the plan snapshot bar at the top correctly displays "0 directives"
8. Click the other snapshots and verify that each one correctly displays the number of directives

[AaronPlave]

The fix here works though seeing a related issue that perhaps should be in a different PR - when I load the first snapshot with no directives the timeline and table still shows the current state of the activities. When I restore the snapshot with no directives it does clear the directives, though the problem reoccurs when I add more directives and repeat the switching of snapshots.

[duranb]

@AaronPlave I think I fixed the issue. It should be ready for another look whenever you have a moment. Thanks!