lsp: Remove vim.NIL handling from apply_text_document_edit

The rpc layer normalizes `vim.NIL` to `nil`, so the scenario tested
should never happen.

runtime/lua/vim/lsp/util.lua

@@ -470,7 +470,6 @@ function M.apply_text_document_edit(text_document_edit, index)
   -- `VersionedTextDocumentIdentifier`s version may be null
   --  https://microsoft.github.io/language-server-protocol/specification#versionedTextDocumentIdentifier
   if should_check_version and (text_document.version
-      and text_document.version ~= vim.NIL
       and text_document.version > 0
       and M.buf_versions[bufnr]
       and M.buf_versions[bufnr] > text_document.version) then

test/functional/plugin/lsp_spec.lua

@@ -1137,19 +1137,6 @@ describe('LSP', function()
         '2nd line of 语text';
       }, buf_lines(target_bufnr))
     end)
-    it('correctly goes ahead with the edit if the version is vim.NIL', function()
-      -- we get vim.NIL when we decode json null value.
-      local json = exec_lua[[
-        return vim.fn.json_decode("{ \"a\": 1, \"b\": null }")
-      ]]
-      eq(json.b, exec_lua("return vim.NIL"))
-
-      exec_lua('vim.lsp.util.apply_text_document_edit(...)', text_document_edit(exec_lua("return vim.NIL")))
-      eq({
-        'First ↥ 🤦 🦄 line of text';
-        '2nd line of 语text';
-      }, buf_lines(target_bufnr))
-    end)
     it('skips the edit if the version of the edit is behind the local buffer ', function()
       local apply_edit_mocking_current_version = function(edit, versionedBuf)
         exec_lua([[