Allow construction of 0x0 Tridiagonal matrix (fixes #26377)

[goggle]

This PR solves the last remaining issue of #26377. Prior to this PR it was not possible to display a Bunch-Kaufmann decomposition of an empty matrix, because we could not construct empty tridiagonal matrices:

julia> bunchkaufman(ones(0,0))
BunchKaufman{Float64,Array{Float64,2}}
D factor:
Error showing value of type BunchKaufman{Float64,Array{Float64,2}}:
ERROR: ArgumentError: cannot construct Tridiagonal from incompatible lengths of subdiagonal, diagonal and superdiagonal: (0, 0, 0)
Stacktrace:
 [1] Type at /build/julia/src/julia-1.2.0/usr/share/julia/stdlib/v1.2/LinearAlgebra/src/tridiag.jl:384 [inlined]
 [2] Type at /build/julia/src/julia-1.2.0/usr/share/julia/stdlib/v1.2/LinearAlgebra/src/tridiag.jl:423 [inlined]
 [3] getproperty(::BunchKaufman{Float64,Array{Float64,2}}, ::Symbol) at /build/julia/src/julia-1.2.0/usr/share/julia/stdlib/v1.2/LinearAlgebra/src/bunchkaufman.jl:231
 [4] show(::IOContext{REPL.Terminals.TTYTerminal}, ::MIME{Symbol("text/plain")}, ::BunchKaufman{Float64,Array{Float64,2}}) at /build/julia/src/julia-1.2.0/usr/share/julia/stdlib/v1.2/LinearAlgebra/src/bunchkaufman.jl:260
 [5] #invokelatest#1 at ./essentials.jl:790 [inlined]
 [6] invokelatest at ./essentials.jl:789 [inlined]
 [7] print_response(::IO, ::Any, ::Bool, ::Bool, ::Any) at /build/julia/src/julia-1.2.0/usr/share/julia/stdlib/v1.2/REPL/src/REPL.jl:156

After this PR:

julia> bunchkaufman(ones(0,0))
BunchKaufman{Float64,Array{Float64,2}}
D factor:
0×0 Tridiagonal{Float64,Array{Float64,1}}
U factor:
0×0 UnitUpperTriangular{Float64,Array{Float64,2}}
permutation:
0-element Array{Int64,1}

Furthermore, I have added tests for LU and Bunch-Kaufmann decompositions of 0x0 matrices.

All the other mentioned issues in #26377 seem to be solved:

julia> lu(ones(0,0))
LU{Float64,Array{Float64,2}}
L factor:
0×0 Array{Float64,2}
U factor:
0×0 Array{Float64,2}

julia> tril(ones(0,0), -1)
0×0 Array{Float64,2}

julia> tril(ones(0,0), 0)
0×0 Array{Float64,2}

julia> triu(ones(0,0), 1)
0×0 Array{Float64,2}

julia> triu(ones(0,0), 0)
0×0 Array{Float64,2}