Feature Request: Singleton type support

Currently:

julia> StructArray([nothing, nothing])
ERROR: ArgumentError: only eltypes with fields are supported
Stacktrace:
 [1] (StructVector{Nothing, @NamedTuple{}})(c::@NamedTuple{})
   @ StructArrays ~/.julia/packages/StructArrays/bgm9N/src/structarray.jl:17
 [2] (StructArray{Nothing})(c::@NamedTuple{})
   @ StructArrays ~/.julia/packages/StructArrays/bgm9N/src/structarray.jl:94
 [3] buildfromschema(initializer::StructArrays.var"#28#29"{…}, ::Type{…}, ::Type{…})
   @ StructArrays ~/.julia/packages/StructArrays/bgm9N/src/utils.jl:48
 [4] buildfromschema(initializer::StructArrays.var"#28#29"{typeof(StructArrays.alwaysfalse), Vector{Nothing}}, ::Type{Nothing})
   @ StructArrays ~/.julia/packages/StructArrays/bgm9N/src/utils.jl:35
 [5] similar_structarray(v::Vector{Nothing}, ::Type{Nothing}; unwrap::typeof(StructArrays.alwaysfalse))
   @ StructArrays ~/.julia/packages/StructArrays/bgm9N/src/structarray.jl:182
 [6] StructArray(v::Vector{Nothing}; unwrap::typeof(StructArrays.alwaysfalse))
   @ StructArrays ~/.julia/packages/StructArrays/bgm9N/src/structarray.jl:260
 [7] StructArray(v::Vector{Nothing})
   @ StructArrays ~/.julia/packages/StructArrays/bgm9N/src/structarray.jl:259
 [8] top-level scope
   @ REPL[43]:1
Some type information was truncated. Use `show(err)` to see complete types.

This feels like an unnecessary limitation. I have an application where I want to be able to store collections of StructArrays filled with concrete types that may or may not be singleton types. It's inconvenient for me to have to fall back on regular vectors for singleton structs.

Reproduce the singleton-type case from the issue in the REPL, then read src/structarray.jl and src/utils.jl around the reported constructors and buildfromschema calls. Done means StructArray([nothing, nothing]) and collections containing singleton and non-singleton concrete types work without falling back to regular vectors.