UniformScaling: hcat vcat and hvcat with matrices of zero size

[mfalt]

Concatenating a UniformScaling with matrix with zero sizes throws error

using LinearAlgebra
A = randn(0,2)
[A I]
ERROR: ArgumentError: hcat of only UniformScaling objects cannot determine the matrix size

B = randn(2,0)
[B; I]
ERROR: ArgumentError: vcat of only UniformScaling objects cannot determine the matrix size

[A I; I I]
ERROR: ArgumentError: sizes of UniformScalings could not be inferred

I would argue that this is a bug and not only a missing implementation since the error message indicates for example hcat of only UniformScaling when this is not the case.

The problem is that 0 is used in cat/hcat/hvcat to indicate unknown sizes. PR: #29457

[fredrikekre]

#29457