add another nothing check to initialize_request

[pfitzseb]

Fixes a bug that showed up on azure:

MethodError: Cannot `convert` an object of type Nothing to an object of type String
Closest candidates are:
  convert(::Type{String}, !Matched::FilePathsBase.AbstractPath) at /Users/.../.vscode/extensions/julialang.language-julia-0.17.8/scripts/packages/FilePathsBase/src/path.jl:107
  convert(::Type{T}, !Matched::T) where T<:AbstractString at strings/basic.jl:209
  convert(::Type{T}, !Matched::AbstractString) where T<:AbstractString at strings/basic.jl:210
   at setindex!(::Dict{String,Nothing}, ::Nothing, ::Nothing) (dict.jl372)
   at push!(::Set{String}, ::Nothing) (set.jl48)
   at initialize_request(::LanguageServer.InitializeParams, ::LanguageServerInstance, ::JSONRPC.JSONRPCEndpoint) (./julialang.language-julia-0.17.8/scripts/packages/LanguageServer/src/requests/init.jl133)
   at (::LanguageServer.var"#111#143"{LanguageServerInstance})(::JSONRPC.JSONRPCEndpoint, ::LanguageServer.InitializeParams) (./julialang.language-julia-0.17.8/scripts/packages/LanguageServer/src/languageserverinstance.jl284)
   at dispatch_msg(::JSONRPC.JSONRPCEndpoint, ::JSONRPC.MsgDispatcher, ::Dict{String,Any}) (./julialang.language-julia-0.17.8/scripts/packages/JSONRPC/src/typed.jl66)
   at run(::LanguageServerInstance) (./julialang.language-julia-0.17.8/scripts/packages/LanguageServer/src/languageserverinstance.jl309)
   at top-level scope (./julialang.language-julia-0.17.8/scripts/languageserver/main.jl59)
   at include(::Module, ::String) (Base.jl377)
   at exec_options(::Base.JLOptions) (client.jl288)
   at _start() (client.jl484)

[davidanthoff]

I think we can probably merge this for now, but in general we don't really have a good story for how to handle non file: URIs. But the problem of course is that there really isn't, we can't really implement these non-standard URI schemes... I think really the LSP should add an ability for us to retrieve file content so that we don't have to interact with the file system at all...