Do not print delimiters twice in show(::OffsetMatrix) for matrices with certain shifted axes

[jishnub]

Currently the _show_nonempty method for an AbstactMatrix had some assumptions about 1-based indexing that did not hold true for OffsetArrays. This led to outputs such as

julia> X = OffsetArray(reshape(1:3, 3, 1), -1:1, 0:0);

julia> show(X)
[1; ; 2; ; 3]

This PR attempts to fix the double delimiters. After this:

julia> show(X)
[1; 2; 3]

and in general show(X) should be identical to show(parent(X))

[jishnub]

Test failure seems unrelated