fix diff generation for new files

Some edits returned by the language server create new files. The URI
property points to a path in the filesystem that does not exist yet. As result
the function to read a line from such a file will return `nil`. While this has
been partially handled within the diff generation it wasn't everywhere. The
consequence was an error propagated to the user and a broken menu.

Author: weilbith

lua/code_action_menu/lsp_objects/edits/text_document_edit.lua

@@ -78,7 +78,8 @@ local function get_list_of_added_lines_in_edit(uri, edit)
   local first_original_complete_line = vim.lsp.util.get_line(
     uri,
     edit.range.start.line
-  )
+  ) or ''
+
   local text_before_changes = first_original_complete_line:sub(
     0,
     edit.range.start.character
@@ -88,7 +89,7 @@ local function get_list_of_added_lines_in_edit(uri, edit)
   local last_original_complete_line = vim.lsp.util.get_line(
     uri,
     edit.range['end'].line
-  )
+  ) or ''
 
   if #last_original_complete_line > edit.range['end'].character then
     local text_after_changes = last_original_complete_line:sub(