Fix reshaping ::Nothing for matrix shapes

[odow]

Found writing #3987

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (b4fc548) to head (c9e1b7a).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #3988   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           43        43           
  Lines         6081      6082    +1     
=========================================
+ Hits          6081      6082    +1     

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[blegat]

Can't we do reshape_vector(::Nothing, ::AbstractShape) = nothing ? If this edge-case was missed, it is surely missed also by extensions that defines shapes (e.g. PolyJuMP) so defining for AbstractShape would be nice