Handle empty blocks upon `mortar`

Nobody has claimed this yet.

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).

Reproduce the example in the Julia REPL, then trace the failure through src/blockaxis.jl, src/views.jl, and src/blockbroadcast.jl as shown in the stacktrace. Determine and document the intended behavior for empty blocks—rejecting them or removing them—and ensure broadcasting a mortared vector no longer raises this BoundsError.

Written by the indexing model from the issue text.

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