Error with matrix_colors and BandedBlockBandedMatrix

[matthieugomez]

matrix_colors cannot compute the colors of this BandedBlockBandedMatrix

using BlockBandedMatrices, SparseDiffTools
l = 4
B = BandedBlockBandedMatrix(Zeros(2 * l , 2 * l), ([l, l] ,[l, l]), (1, 1), (1, 1))
matrix_colors(B)
#> ERROR: ArgumentError: reducing over an empty collection is not allowed

(I think it is a valid BandedBlockBandedMatrix so it should work)

[ChrisRackauckas]

@huanglangwen

[huanglangwen]

#45