allow for empty blocks in left/right-orth

[maartenvd]

at the moment, the following is inconsistent:

    t = TensorMap(zeros,ComplexF64,Rep[SU₂](1=>10)*Rep[SU₂](1=>2),Rep[SU₂](1=>10));
    #(l,q) = rightorth(t,alg=LQpos()); #fails

    data = convert(Array,t);
    tt = TensorMap(data,ℂ^size(data,1)*ℂ^size(data,2),ℂ^size(data,3))
    (l,q) = rightorth(tt,alg=LQpos()); #works

In this pull request they now work the same way - by making l rank-defficient, while q remains unitary.

[codecov]

Codecov Report

Merging #49 (be3dd61) into master (846a68e) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
+ Coverage   74.96%   74.98%   +0.01%     
==========================================
  Files          32       32              
  Lines        3907     3909       +2     
==========================================
+ Hits         2929     2931       +2     
  Misses        978      978              

Impacted Files	Coverage Δ
src/tensors/factorizations.jl	95.54% <100.00%> (+0.04%)	⬆️

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[Jutho]

Look's good; thanks.