lsp: remove vim.NIL from processing (#13174)

* lsp: remove vim.NIL from processing

* lsp: remove instances of vim.NIL

runtime/lua/vim/lsp/rpc.lua

@@ -42,13 +42,28 @@ local function is_dir(filename)
 end
 
 local NIL = vim.NIL
+
+--@private
+local recursive_convert_NIL
+recursive_convert_NIL = function(v, tbl_processed)
+  if v == NIL then
+    return nil
+  elseif not tbl_processed[v] and type(v) == 'table' then
+    tbl_processed[v] = true
+    return vim.tbl_map(function(x)
+      return recursive_convert_NIL(x, tbl_processed)
+    end, v)
+  end
+
+  return v
+end
+
 --@private
 --- Returns its argument, but converts `vim.NIL` to Lua `nil`.
 --@param v (any) Argument
 --@returns (any)
 local function convert_NIL(v)
-  if v == NIL then return nil end
-  return v
+  return recursive_convert_NIL(v, {})
 end
 
 --@private