build_tree errors with unparsed ParseStream

Broke sometime after eceaa39. We used to get an empty K"wrapper", but I'm not sure what the best thing to do is, since it's possible the user just forgot to `parse!`. (Not always; JETLS was using this code to produce a type and was OK with the wrapper.)

```
JS.build_tree(JS.SyntaxNode, JS.ParseStream(""))

ERROR: BoundsError: attempt to access 1-element Vector{JuliaSyntax.RawGreenNode} at index [0]
Stacktrace:
[1] throw_boundserror(A::Vector{JuliaSyntax.RawGreenNode}, I::Tuple{Int64})
@ Base ./essentials.jl:15
[2] checkbounds
@ ./essentials.jl:387 [inlined]
[3] getindex
@ ./essentials.jl:966 [inlined]
[4] getindex
@ ./abstractarray.jl:1358 [inlined]
[5] this
@ ~/.julia/packages/JuliaSyntax/4IHpz/src/core/tree_cursors.jl:25 [inlined]
[6] head
@ ~/.julia/packages/JuliaSyntax/4IHpz/src/core/tree_cursors.jl:70 [inlined]
[7] JuliaSyntax.GreenNode(cursor::JuliaSyntax.GreenTreeCursor)
@ JuliaSyntax ~/.julia/packages/JuliaSyntax/4IHpz/src/porcelain/green_node.jl:122
[8] build_tree(::Type{…}, stream::JuliaSyntax.ParseStream; filename::Nothing, first_line::Int64, keep_parens::Bool, kws::@Kwargs{})
@ JuliaSyntax ~/.julia/packages/JuliaSyntax/4IHpz/src/porcelain/syntax_tree.jl:315
[9] build_tree(::Type{SyntaxNode}, stream::JuliaSyntax.ParseStream)
@ JuliaSyntax ~/.julia/packages/JuliaSyntax/4IHpz/src/porcelain/syntax_tree.jl:304

```