`dimension` fetches fist item of `AbstractArray` even if empty

While wrapping `QuantityArray`s with `KeyedArray` from [`AxisKeys.jl`](https://github.com/mcabbott/AxisKeys.jl), I met an edge case of `dimension` with empty arrays.

```julia
] activate --temp
] add AxisKeys
] add DynamicQuantities

A = KeyedArray(QuantityArray(Matrix{Float64}(undef,0,6),us"s"); time = Float64[], channel = ["$i" for i in 1:6])
dimension(A)
```
This throws
```
julia> dimension(A)
ERROR: BoundsError: attempt to access 0×6 NamedDimsArray(QuantityArray(::Matrix{Float64}, ::Quantity{Float64, SymbolicDimensions{FRInt32}}), (:time, :channel)) at index [1]
Stacktrace:
[1] throw_boundserror(A::NamedDimsArray{(:time, :channel), Quantity{…}, 2, QuantityArray{…}}, I::Tuple{Int64})
@ Base .\essentials.jl:15
[2] checkbounds
@ .\abstractarray.jl:699 [inlined]
[3] getindex
@ D:\bamboo\.julia\packages\AxisKeys\a6h2b\src\struct.jl:76 [inlined]
[4] first
@ .\abstractarray.jl:452 [inlined]
[5] dimension(aq::KeyedArray{Quantity{…}, 2, NamedDimsArray{…}, Tuple{…}})
@ DynamicQuantities D:\bamboo\.julia\packages\DynamicQuantities\wqpWc\src\utils.jl:442
[6] top-level scope
@ REPL[26]:1
Some type information was truncated. Use `show(err)` to see complete types.
```
at https://github.com/JuliaPhysics/DynamicQuantities.jl/blob/f619e165815124d66be7135c495352f769afcdc8/src/utils.jl#L442

I am not sure how this should be circumvented. If the array is empty, it is obvious that all elements should be of the same type. It may require an additional dispatch.