Fix IsomorphismReesZeroMatrixSemigroup

[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)

For a finite 0-simple semigroup, previously the isomorphism returned by IsomorphismReesZeroMatrixSemigroup was not defined on the zero of its domain or range.

[codecov-io]

Current coverage is 49.56% (diff: 37.50%)

Merging #1038 into master will increase coverage by <.01%

@@             master      #1038   diff @@
==========================================
  Files           424        424          
  Lines        223277     223287    +10   
  Methods        3430       3430          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits         110656     110683    +27   
+ Misses       112621     112604    -17   
  Partials          0          0          

Powered by Codecov. Last update 4ff9b38...b947738

[fingolfin]

Thanks!