Fix bug in MeetOfPartialPerms

[james-d-mitchell]

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

Tick all what applies to this pull request

• Adds new features
• Improves and extends functionality
• Fixes bugs that could lead to crashes
• Fixes bugs that could lead to incorrect results
• Fixes bugs that could lead to break loops

Write below the description of changes (for the release notes)

Previously if the argument was empty or of length 1, then this function
would error by trying to access unbound list entries. This commit fixes this, and adds tests.

[codecov-io]

Current coverage is 49.58% (diff: 100%)

Merging #1035 into master will increase coverage by 0.05%

@@             master      #1035   diff @@
==========================================
  Files           424        424          
  Lines        223274     223277     +3   
  Methods        3430       3430          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits         110593     110710   +117   
+ Misses       112681     112567   -114   
  Partials          0          0          

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