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.

autoload/vimwiki/diary.vim

@@ -523,7 +523,7 @@ endfunction
 function! vimwiki#diary#calendar_sign(day, month, year) abort
   " Callback function for Calendar.vim
   " Clause: no wiki no sign #290
-  if len(g:vimwiki_list) <= 0
+  if exists('g:vimwiki_list') && len(g:vimwiki_list) <= 0
     return
   endif
   let day = s:prefix_zero(a:day)

doc/vimwiki.txt

@@ -4002,6 +4002,7 @@ Contributors and their Github usernames in roughly chronological order:
     - Thomas Leyh (@leyhline)
     - nebulaeandstars (@nebulaeandstars)
     - dmitry kim (@jsn)
+    - Luke Atkinson (@LukeDAtkinson)
 
 ==============================================================================
 16. Changelog                                              *vimwiki-changelog*
@@ -4021,6 +4022,12 @@ This is somewhat experimental, and will probably be refined over time.
 
 2023.04.04~
 
+Fixed:~
+    * Issue #1336: vimwiki#diary#calendar_sign throws an error
+      when g:vimwiki_list is not set
+ 
+2023.04.04~
+
 New:~
     * Issue #1261: Feature: Support for <mark> in Markdown
       highlighting with yellow background and back foreground

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')

test/tag.vader

@@ -257,7 +257,7 @@ Expect (Correctly formatted tags file):
   !_TAG_PROGRAM_AUTHOR	Vimwiki
   !_TAG_PROGRAM_NAME	Vimwiki Tags
   !_TAG_PROGRAM_URL	https://github.com/vimwiki/vimwiki
-  !_TAG_PROGRAM_VERSION	2023.04.04
+  !_TAG_PROGRAM_VERSION	2023.05.12
   second-tag	Test-Tag.md	13;"	vimwiki:Test-Tag\tTest-Tag#second-tag\tTest-Tag#second-tag
   test-tag	Test-Tag.md	5;"	vimwiki:Test-Tag\tTest-Tag#a-header\tA header
   top-tag	Test-Tag.md	1;"	vimwiki:Test-Tag\tTest-Tag\tTest-Tag