Make blockdiag return a 0×0 SparseMatrixCSC{Union{},Int}

[martinholters]

Fixes #14850.

[martinholters]

All CI failures look unrelated:

• Circle: "ErrorException("_length is deprecated, use length instead.")" all over the place. I guess Circle ran after Support arbitrary axes for ReinterpretArray #27941 and Remove the "experimental" status from non-1 indexing #24899, but before Fix _length in reinterpretarray #28068.
• AV 64bit:

┌ Error: mktempdir cleanup
│   exception =
│    unlink: resource busy or locked (EBUSY)
│    Stacktrace:
│     [1] uv_error at .\libuv.jl:80 [inlined]

during Pkg test.

• FreeBSD: httpbin problem.

[andreasnoack]

Actually, will this cause a type stability issue if the index types of the inputs are non-Ints?

[martinholters]

No, the isempty(X) and the conditional is constant-folded (X is a tuple with length known at compile time). Well, unless the number of arguments is unknown at compile time, but the type instability seems unavoidable then.