Fix show method for pseudo-matrices

[SirToby25]

Previously, generating a pseudo-matrix over an algebraic number field with zero rows was possible, e.g.,

K, _ = quadratic_field(-5)
P = pseudo_matrix(identity_matrix(K, 0), [])

and the pseudo-matrix P was created correctly. However, an error was raised, due to an undefined reference and this P could not be printed. This commit just adds an extra output for pseudo-matrices with zero rows.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 75.99%. Comparing base (5b4309e) to head (3fe5f78).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/NumFieldOrd/NfOrd/LinearAlgebra.jl	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1689       +/-   ##
===========================================
+ Coverage    5.68%   75.99%   +70.31%     
===========================================
  Files         362      362               
  Lines      114128   114327      +199     
===========================================
+ Hits         6485    86884    +80399     
+ Misses     107643    27443    -80200     

Files with missing lines	Coverage Δ
src/NumFieldOrd/NfOrd/LinearAlgebra.jl	57.25% <0.00%> (+57.11%)	⬆️

... and 322 files with indirect coverage changes

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[thofma]

Thanks