Coefficient type determination for an empty polynomial matrix

This is possibly a silly question, but is related to the previous issue [#308](https://github.com/JuliaMath/Polynomials.jl/issues/308) .

Admitedly we have an empty polynomial matrix, created for example with the command:
```
julia> a=Polynomial.(rand(0,0))
0×0 Array{Polynomial{Float64},2}

julia> eltype(a)
Polynomial{Float64}
```
how it is possible to determine the type of coefficients (in the above case `Float64`).
For a nonempty matrix, this is easy

```
julia> a=Polynomial.(rand(1,1))
1×1 Array{Polynomial{Float64},2}:
Polynomial(0.8736304322844237)

julia> eltype(a[1])
Float64
```

Start by reproducing the Julia commands in the issue and compare the types of empty and nonempty polynomial matrices. Read the existing polynomial and matrix type-handling entry points; done means there is a clear, supported way to determine the coefficient type for an empty polynomial matrix, with behavior explained or covered by tests.

Written by the indexing model from the issue text.