Fix converting empty tensors to array

[lkdvos]

This PR fixes #178

[Jutho]

It would be great if you could also add Base.zero(V::ElementarySpace) on line 92 of "docs/src/lib/spaces.md". (If current CI has already finished, this can be with [skip ci]; the docs will be built when this is merged.)

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.80%. Comparing base (f467a21) to head (d594dec).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/spaces/generalspace.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #180      +/-   ##
==========================================
+ Coverage   81.39%   82.80%   +1.41%     
==========================================
  Files          42       42              
  Lines        5218     5212       -6     
==========================================
+ Hits         4247     4316      +69     
+ Misses        971      896      -75     

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