Fix empty Tridiagonal broadcast

[jishnub]

Fixes

julia> T = Tridiagonal(1:0, 1:0, 1:0)
0×0 Tridiagonal{Int64, UnitRange{Int64}}

julia> T .+ T
ERROR: ArgumentError: invalid GenericMemory size: the number of elements is either negative or too large for system address width

After this,

julia> T .+ T
0×0 Tridiagonal{Int64, Vector{Int64}}

The changes are minor, but the largish diff is because of adding a loop to the tests, and additional indentation as a consequence.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.74%. Comparing base (41db513) to head (7c66b6b).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1324      +/-   ##
==========================================
+ Coverage   93.73%   93.74%   +0.01%     
==========================================
  Files          34       34              
  Lines       15750    15750              
==========================================
+ Hits        14763    14765       +2     
+ Misses        987      985       -2     

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