Remove base_ring methods returning Union{}

[fingolfin]

This is certainly a breaking change, in that it changes an explicitly documented behavior. But I suspect it won't actually break anything ; and if it does, I am glad to learn about it.

Rationale for this:

1. I suspect it is the cause of some JET warnings
2. returning the type Union{} for base_ring never made sense to me. Besides not fitting logically (a type is not a ring...): When would it even be used, and why?
3. even if something relies on testing that a given ring "has no base ring" (but why???) then it makes more sense to let base_ring_type return Union{} (a method which did not exist when the "base_ring may return Union{}" convention was introduced)

Alas, I was not there when this was was designed and perhaps there are serious downsides I am not aware of -- well, as usual I am happy to learn :-).

If we merge this then of course Nemo, Hecke, ... need some changes (but I think mostly of the variety "drop base_ring method returning Union{} and adjust its base_ring_type method" )

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.96%. Comparing base (fea987d) to head (98a41d5).

Files with missing lines	Patch %	Lines
src/julia/Integer.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2144      +/-   ##
==========================================
- Coverage   87.97%   87.96%   -0.02%     
==========================================
  Files         126      126              
  Lines       31672    31669       -3     
==========================================
- Hits        27864    27858       -6     
- Misses       3808     3811       +3     

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[thofma]

No objection from my side.

[fieker]

as long as s.o. makes sure the system continues to work... I never was a fan of Union{}, so be my guest