prevent beeps when navigating multi-line commands

`man zshzle` at the "zle widget" section:

A non-zero return status causes the shell to beep when the widget exits,
unless the BEEP options was unset or the widget was called via the zle
command. Thus if a user defined widget requires an immediate beep, it
should call the beep widget directly.

https://github.com/robbyrussell/oh-my-zsh/pull/215#issuecomment-1328406

history-substring-search.zsh

@@ -112,6 +112,8 @@ history-substring-search-end() {
   # for debugging purposes:
   #zle -R "mn: "$history_substring_search_match_number" m#: "${#history_substring_search_matches}
   #read -k -t 200 && zle -U $REPLY
+
+  true
 }
 
 history-substring-search-backward() {