fix: sort_by "extension" falls back to name (#2306)

* fix(extension/sorter): fallbacks to C.name when both exts are the same or nil

* fix(nil): files with no extension

* fix(nil): files with no extension

---------

Co-authored-by: Alexander Courtis <alex@courtis.org>

Author: moniquelive

lua/nvim-tree/explorer/sorters.lua

@@ -179,17 +179,19 @@ function C.extension(a, b)
     return false
   end
 
-  if not (a.extension and b.extension) then
-    return true
-  end
-
   if a.extension and not b.extension then
     return true
   elseif not a.extension and b.extension then
     return false
   end
 
-  return a.extension:lower() <= b.extension:lower()
+  local a_ext = (a.extension or ""):lower()
+  local b_ext = (b.extension or ""):lower()
+  if a_ext == b_ext then
+    return C.name(a, b)
+  end
+
+  return a_ext < b_ext
 end
 
 function C.filetype(a, b)