Handle empty blocks upon `mortar`

[lassepe]

I just ran into the following behavior (due to another bug in my own code):

julia> a = mortar([Bool[], Bool[], [true, false]])
julia> a .|| a
ERROR: BoundsError: attempt to access 0-element UnitRange{Int64} at index [1:2]

Full Stacktrace

Stacktrace:
  [1] throw_boundserror(A::UnitRange{Int64}, I::Tuple{UnitRange{Int64}})
    @ Base ./abstractarray.jl:691
  [2] checkbounds
    @ ./abstractarray.jl:656 [inlined]
  [3] getindex
    @ ./range.jl:918 [inlined]
  [4] getindex
    @ ~/.julia/packages/BlockArrays/tr499/src/blockaxis.jl:10 [inlined]
  [5] unblock
    @ ~/.julia/packages/BlockArrays/tr499/src/views.jl:10 [inlined]
  [6] to_indices
    @ ~/.julia/packages/BlockArrays/tr499/src/views.jl:28 [inlined]
  [7] to_indices
    @ ~/.julia/packages/BlockArrays/tr499/src/views.jl:43 [inlined]
  [8] view(A::BlockVector{Bool, Vector{Vector{Bool}}, Tuple{BlockedUnitRange{Vector{Int64}}}}, I::BlockArrays.BlockIndexRange{1, Tuple{UnitRange{Int64}}})
    @ Base ./subarray.jl:176
  [9] _bview
    @ ~/.julia/packages/BlockArrays/tr499/src/blockbroadcast.jl:145 [inlined]
 [10] macro expansion
    @ ~/.julia/packages/BlockArrays/tr499/src/blockbroadcast.jl:175 [inlined]
 [11] _generic_blockbroadcast_copyto!(dest::BlockVector{Bool, Vector{Vector{Bool}}, Tuple{BlockedUnitRange{Vector{Int64}}}}, bc::Base.Broadcast.Broadcasted{BlockArrays.BlockStyle{1}, Tuple{BlockedUnitRange{Vector{Int64}}}, Base.Broadcast.var"#5#6", Tuple{BlockVector{Bool, Vector{Vector{Bool}}, Tuple{BlockedUnitRange{Vector{Int64}}}}, BlockVector{Bool, Vector{Vector{Bool}}, Tuple{BlockedUnitRange{Vector{Int64}}}}}})
    @ BlockArrays ~/.julia/packages/BlockArrays/tr499/src/blockbroadcast.jl:151
 [12] copyto!(dest::BlockVector{Bool, Vector{Vector{Bool}}, Tuple{BlockedUnitRange{Vector{Int64}}}}, bc::Base.Broadcast.Broadcasted{BlockArrays.BlockStyle{1}, Tuple{BlockedUnitRange{Vector{Int64}}}, Base.Broadcast.var"#5#6", Tuple{BlockVector{Bool, Vector{Vector{Bool}}, Tuple{BlockedUnitRange{Vector{Int64}}}}, BlockVector{Bool, Vector{Vector{Bool}}, Tuple{BlockedUnitRange{Vector{Int64}}}}}})
    @ BlockArrays ~/.julia/packages/BlockArrays/tr499/src/blockbroadcast.jl:219
 [13] copy
    @ ./broadcast.jl:885 [inlined]
 [14] materialize(bc::Base.Broadcast.Broadcasted{BlockArrays.BlockStyle{1}, Nothing, Base.Broadcast.var"#5#6", Tuple{BlockVector{Bool, Vector{Vector{Bool}}, Tuple{BlockedUnitRange{Vector{Int64}}}}, BlockVector{Bool, Vector{Vector{Bool}}, Tuple{BlockedUnitRange{Vector{Int64}}}}}})
    @ Base.Broadcast ./broadcast.jl:860
 [15] top-level scope
    @ REPL[137]:1
 [16] top-level scope
    @ ~/.julia/packages/Infiltrator/WQe1n/src/Infiltrator.jl:508
 [17] top-level scope

It was surprisingly annoying to find where this was coming from since the faulty a::BlockVector{Bool} in many ways behaves normally but broadcasting then fails. It may be a good idea to just throw an error upon mortar with empty blocks or eliminate them silently (though that may cause other surprises, and, in my case, would have hidden the actual error that I had).

[dlfivefifty]

I think empty blocks are probably useful… so would probably be best to figure out how to fix broadcasting

[lassepe]

That's fine with me as well. Just thought that the current behavior is probably a bug.

[lassepe]

I should probably add that his does not happen for arbitrary broadcasting. a .+ 1 for the vector above is fine, for example. Similarly, a = mortar(Bool[], [true, false]) (with only a single empty block) does not fail upon a .|| a.