Unexpected Errors using 'prod()' when specifying 'dims = _' argument on matrices of Unitful values

Hello,

today I observed a couple of unexpected behaviours using the `Unitful.jl` package - I'm not sure what to make of them, since it is quite easy to work around the errors as seen in the MWE below. Nevertheless, I thought it would still be worth sharing.

I have included the stack traces below the MWE for both the `DimensionError` and the `ArgumentError`.

In the case of the `DimensionError`, this seems on the surface to be similar to #585.

In the case of the `ArgumentError`, I'm confused at this behaviour and I haven't seen anything similar in the other issues (I might have missed something, though).

Thanks for your time.

J

----
MWE

```
using Unitful

A = [1 2; 3 4]

prod(A) # 24
A[1, :] .* A[2, :] # [3, 8]
prod(A; dims = 1) # [3, 8]

B = A .* u"m"

prod(B) # 24 m⁴
B[1, :] .* B[2, :] # [3 m², 8 m²]
prod(B; dims = 1) # DimensionError: and m² are not dimensionally compatible.

C = A .* [u"m" u"m"; u"kg" u"kg"]

prod(C) # 24 kg² m²
C[1, :] .* C[2, :] # [3 kg m, 8 kg m]
prod(C; dims = 1) # ArgumentError: zero(Quantity{Int64}) not defined.

```
----
Version information

Unitful:
```
Status `~/.julia/environments/v1.9/Project.toml`
[1986cc42] Unitful v1.19.0

julia> versioninfo()
Julia Version 1.9.4
Commit 8e5136fa297 (2023-11-14 08:46 UTC)
Build Info:
Official https://julialang.org/ release
Platform Info:
OS: macOS (x86_64-apple-darwin22.4.0)
CPU: 4 × Intel(R) Core(TM) i5-5250U CPU @ 1.60GHz
WORD_SIZE: 64
LIBM: libopenlibm
LLVM: libLLVM-14.0.6 (ORCJIT, broadwell)
Threads: 1 on 4 virtual cores
Environment:
JULIA_EDITOR = code
JULIA_NUM_THREADS =
```
----

Stacktraces

`DimensionError`
```
ERROR: DimensionError: and m² are not dimensionally compatible.
Stacktrace:
[1] #s103#141
@ ~/.julia/packages/Unitful/R4J37/src/conversion.jl:7 [inlined]
[2] var"#s103#141"(::Any, s::Any, t::Any)
@ Unitful ./none:0
[3] (::Core.GeneratedFunctionStub)(::Any, ::Vararg{Any})
@ Core ./boot.jl:602
[4] uconvert(a::Unitful.FreeUnits{(), NoDims, nothing}, x::Quantity{Int64, 𝐋², Unitful.FreeUnits{(m²,), 𝐋², nothing}})
@ Unitful ~/.julia/packages/Unitful/R4J37/src/conversion.jl:72
[5] convert(#unused#::Type{Int64}, y::Quantity{Int64, 𝐋², Unitful.FreeUnits{(m²,), 𝐋², nothing}})
@ Unitful ~/.julia/packages/Unitful/R4J37/src/conversion.jl:139
[6] setindex!
@ ./array.jl:971 [inlined]
[7] setindex!
@ ./multidimensional.jl:670 [inlined]
[8] _mapreducedim!(f::typeof(identity), op::typeof(Base.mul_prod), R::Matrix{Int64}, A::Matrix{Quantity{Int64, 𝐋, Unitful.FreeUnits{(m,), 𝐋, nothing}}})
@ Base ./reducedim.jl:311
[9] mapreducedim!(f::Function, op::Function, R::Matrix{Int64}, A::Matrix{Quantity{Int64, 𝐋, Unitful.FreeUnits{(m,), 𝐋, nothing}}})
@ Base ./reducedim.jl:324
[10] _mapreduce_dim(f::Function, op::Function, #unused#::Base._InitialValue, A::Matrix{Quantity{Int64, 𝐋, Unitful.FreeUnits{(m,), 𝐋, nothing}}}, dims::Int64)
@ Base ./reducedim.jl:371
[11] #mapreduce#801
@ ./reducedim.jl:357 [inlined]
[12] mapreduce
@ ./reducedim.jl:357 [inlined]
[13] #_prod#839
@ ./reducedim.jl:1023 [inlined]
[14] _prod
@ ./reducedim.jl:1023 [inlined]
[15] #_prod#838
@ ./reducedim.jl:1022 [inlined]
[16] _prod
@ ./reducedim.jl:1022 [inlined]
[17] #prod#812
@ ./reducedim.jl:994 [inlined]
[18] top-level scope
```

`ArgumentError`
```
ERROR: ArgumentError: zero(Quantity{Int64}) not defined.
Stacktrace:
[1] zero(x::Type{Quantity{Int64}})
@ Unitful ~/.julia/packages/Unitful/R4J37/src/quantities.jl:363
[2] _reducedim_init(f::typeof(identity), op::typeof(Base.mul_prod), fv::typeof(one), fop::typeof(prod), A::Matrix{Quantity{Int64}}, region::Int64)
@ Base ./reducedim.jl:116
[3] reducedim_init(f::Function, op::typeof(Base.mul_prod), A::Matrix{Quantity{Int64}}, region::Int64)
@ Base ./reducedim.jl:111
[4] _mapreduce_dim(f::Function, op::Function, #unused#::Base._InitialValue, A::Matrix{Quantity{Int64}}, dims::Int64)
@ Base ./reducedim.jl:371
[5] #mapreduce#801
@ ./reducedim.jl:357 [inlined]
[6] mapreduce
@ ./reducedim.jl:357 [inlined]
[7] #_prod#839
@ ./reducedim.jl:1023 [inlined]
[8] _prod
@ ./reducedim.jl:1023 [inlined]
[9] #_prod#838
@ ./reducedim.jl:1022 [inlined]
[10] _prod
@ ./reducedim.jl:1022 [inlined]
[11] #prod#812
@ ./reducedim.jl:994 [inlined]
[12] top-level scope
```
----

Start by running the MWE with Julia 1.9.4 and Unitful 1.19.0. Trace the calls through Base's reducedim.jl and the Unitful locations shown in conversion.jl and quantities.jl:363. Done means the two prod(B; dims = 1) cases no longer produce the reported errors and retain the expected values and units.

Written by the indexing model from the issue text.