Fix issymmetric for matrices with empty columns

[jishnub]

The issymmetric check tracks an offset that it uses to go from (row, col) to (col, row). However, currently this doesn't account for the fact that if a column is empty, entries in colptr will be identical. E.g., in #605, we have

julia> S = sparse([2, 3, 1], [1, 1, 3], [1, 1, 1], 3, 3)
3×3 SparseMatrixCSC{Int64, Int64} with 3 stored entries:
 ⋅  ⋅  1
 1  ⋅  ⋅
 1  ⋅  ⋅

julia> SparseArrays.getcolptr(S)
4-element Vector{Int64}:
 1
 3
 3
 4

The offset 3 corresponds to rows in the third column, as the second column is empty. This PR checks for empty columns, in which case we may exit the call immediately.

Fixes #605

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.07%. Comparing base (9548149) to head (4303560).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #606      +/-   ##
==========================================
- Coverage   84.10%   84.07%   -0.04%     
==========================================
  Files          12       12              
  Lines        9192     9193       +1     
==========================================
- Hits         7731     7729       -2     
- Misses       1461     1464       +3     

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