add a nothing-check in check_farg_unused

[pfitzseb]

Fixes an error on Azure:

ErrorException: type Nothing has no field refs 
   at getproperty(::Nothing, ::Symbol) (Base.jl33)
   at check_farg_unused(::CSTParser.EXPR) (./julialang.language-julia-insider-1.0.10/scripts/packages/StaticLint/src/linting/checks.jl471)
   at check_all(::CSTParser.EXPR, ::StaticLint.LintOptions, ::LanguageServerInstance) (./julialang.language-julia-insider-1.0.10/scripts/packages/StaticLint/src/linting/checks.jl93)
   at check_all(::CSTParser.EXPR, ::StaticLint.LintOptions, ::LanguageServerInstance) (./julialang.language-julia-insider-1.0.10/scripts/packages/StaticLint/src/linting/checks.jl101)
   at lint!(::LanguageServer.Document, ::LanguageServerInstance) (./julialang.language-julia-insider-1.0.10/scripts/packages/LanguageServer/src/staticlint.jl58)
   at parse_all(::LanguageServer.Document, ::LanguageServerInstance) (./julialang.language-julia-insider-1.0.10/scripts/packages/LanguageServer/src/requests/textdocument.jl238)
   at load_folder(::String, ::LanguageServerInstance) (./julialang.language-julia-insider-1.0.10/scripts/packages/LanguageServer/src/requests/init.jl111)
   at initialized_notification(::LanguageServer.InitializedParams, ::LanguageServerInstance, ::JSONRPC.JSONRPCEndpoint{Base.PipeEndpoint,Base.PipeEndpoint}) (./julialang.language-julia-insider-1.0.10/scripts/packages/LanguageServer/src/requests/init.jl175)
   at (::LanguageServer.var"#112#145"{LanguageServerInstance})(::JSONRPC.JSONRPCEndpoint{Base.PipeEndpoint,Base.PipeEndpoint}, ::LanguageServer.InitializedParams) (./julialang.language-julia-insider-1.0.10/scripts/packages/LanguageServer/src/languageserverinstance.jl285)
   at dispatch_msg(::JSONRPC.JSONRPCEndpoint{Base.PipeEndpoint,Base.PipeEndpoint}, ::JSONRPC.MsgDispatcher, ::Dict{String,Any}) (./julialang.language-julia-insider-1.0.10/scripts/packages/JSONRPC/src/typed.jl66)
   at run(::LanguageServerInstance) (./julialang.language-julia-insider-1.0.10/scripts/packages/LanguageServer/src/languageserverinstance.jl310)
   at top-level scope (./julialang.language-julia-insider-1.0.10/scripts/languageserver/main.jl59)
   at include(::Function, ::Module, ::String) (Base.jl380)
   at include(::Module, ::String) (Base.jl368)
   at exec_options(::Base.JLOptions) (client.jl296)
   at _start() (client.jl506)