Fix some dialyzer errors (elixir-editors#103)

* :cwd option expects a charlist

* Signature Help Request should return null instead of empty array when no signature found

see https://microsoft.github.io/language-server-protocol/specifications/specification-3-15/#textDocument_signatureHelp

* ElixirSense.docs never returns nil

instead return null when no subject is found
see https://microsoft.github.io/language-server-protocol/specifications/specification-3-15/#textDocument_hover

Author: lukaszsamson

apps/elixir_ls_utils/lib/mix.tasks.elixir_ls.release.ex

@@ -33,7 +33,7 @@ defmodule Mix.Tasks.ElixirLs.Release do
     if opts[:zip] do
       zip_file = to_charlist(Path.expand(opts[:zip]))
       files = Enum.map(File.ls!(destination), &to_charlist/1)
-      :zip.create(zip_file, files, cwd: destination)
+      :zip.create(zip_file, files, cwd: to_charlist(destination))
     end
 
     :ok

apps/language_server/lib/language_server/providers/hover.ex

@@ -4,20 +4,23 @@ defmodule ElixirLS.LanguageServer.Providers.Hover do
   """
 
   def hover(text, line, character) do
-    %{subject: subject, docs: docs} = ElixirSense.docs(text, line + 1, character + 1)
+    response =
+      case ElixirSense.docs(text, line + 1, character + 1) do
+        %{subject: ""} ->
+          nil
 
-    line_text = Enum.at(String.split(text, "\n"), line)
-    range = highlight_range(line_text, line, character, subject)
+        %{subject: subject, docs: docs} ->
+          line_text = Enum.at(String.split(text, "\n"), line)
+          range = highlight_range(line_text, line, character, subject)
 
-    {:ok, %{"contents" => contents(docs), "range" => range}}
+          %{"contents" => contents(docs), "range" => range}
+      end
+
+    {:ok, response}
   end
 
   ## Helpers
 
-  defp highlight_range(_line_text, _line, _character, nil) do
-    nil
-  end
-
   defp highlight_range(line_text, line, character, substr) do
     regex_ranges =
       Regex.scan(

apps/language_server/lib/language_server/providers/signature_help.ex

@@ -10,7 +10,7 @@ defmodule ElixirLS.LanguageServer.Providers.SignatureHelp do
           }
 
         :none ->
-          %{"signatures" => []}
+          nil
       end
 
     {:ok, response}