Fix vimwiki#diary#calendar_sign when g:vimwiki_list is not set

If a user is happy with the default vimwiki location, they may not set
the g:vimwiki_list global variable. In this case, the
len(g:vimwiki_list) vimwiki#diary#calendar_sign function throws an
error every time it is called. This function is called for every day
displayed in the calendar which is very noisy. Checking if the variable
exists before doing the length check prevents the error and the rest of
the function works fine if the variable is not set.

plugin/vimwiki.vim

@@ -11,7 +11,7 @@ endif
 let g:loaded_vimwiki = 1
 
 " Set to version number for release:
-let g:vimwiki_version = '2023.04.04'
+let g:vimwiki_version = '2023.05.12'
 
 " Get the directory the script is installed in
 let s:plugin_dir = expand('<sfile>:p:h:h')