Fix issymmetric/ishermitian for block and empty matrices

[jishnub]

Fixes the following discrepancies, accounting for the fact that issymmetric returns true for an empty matrix, and that it acts recursively:

julia> F = Fill([1 2; 3 4], 2, 2);

julia> issymmetric(F)
true

julia> issymmetric(Array(F))
false

julia> F = Fill(im, 0, 0);

julia> ishermitian(F)
false

julia> ishermitian(Array(F))
true

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.90%. Comparing base (9868632) to head (2f56940).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #377   +/-   ##
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  Coverage   99.90%   99.90%           
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  Files           8        8           
  Lines        1110     1110           
=======================================
  Hits         1109     1109           
  Misses          1        1           

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