Fix intersection

[markuspf]

Please make sure that this pull request:

• is submitted to the correct branch (the stable branch is only for bugfixes)
• contains an accurate description of changes for the release notes below
• provides new tests or relies on existing ones
• correctly refers to other issues and related pull requests

Description of changes (for the release notes)

Fix a bug in Intersection if the parameter given is the empty list.

[codecov-io]

Current coverage is 49.16%

Merging #854 into master will decrease coverage by <.01%

@@             master       #854   diff @@
==========================================
  Files           422        422          
  Lines        233097     233100     +3   
  Methods        3479       3479          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits         114599     114596     -3   
- Misses       118498     118504     +6   
  Partials          0          0          

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[ChrisJefferson]

Looks good to me.
Not sure why we are seeing the amount of coverage bounce around so much. Some random number weirdness I think.

[ChrisJefferson]

Will merge this, as it seems fine.

I notice this matches Union, where Union([]) already works (and produces the same answer).