Remove duplicate normalize_path function

Use the function defined in the "common" module.

Move the check for not-nil filename from common.normalize_path
to core.open_doc. In this latter the filename can be nil if a
new unnamed document is created.

data/core/common.lua

@@ -258,7 +258,7 @@ end
 
 
 function common.normalize_path(filename)
-  if filename and PATHSEP == '\\' then
+  if PATHSEP == '\\' then
     filename = filename:gsub('[/\\]', '\\')
     local drive, rem = filename:match('^([a-zA-Z])(:.*)')
     return drive and drive:upper() .. rem or filename