Simplify logic to extract suggestion from between null bytes

Just remove up to and including the first null byte and after and
including the last null byte.

I also looked into using `${${(0)line}[2]}`, but it fails when `$line`
starts with a null byte, since the first split string will be empty and
thus not included in the resulting array.

zsh-autosuggestions.zsh

@@ -594,7 +594,7 @@ _zsh_autosuggest_strategy_completion() {
 		# versions of zsh (older than 5.3), we sometimes get extra bytes after
 		# the second null byte, so trim those off the end.
 		# See http://www.zsh.org/mla/workers/2015/msg03290.html
-		suggestion="${${${(M)line:#*$'\0'*$'\0'*}#*$'\0'}%%$'\0'*}"
+		suggestion="${${line#*$'\0'}%$'\0'*}"
 	} always {
 		# Destroy the pty
 		zpty -d $ZSH_AUTOSUGGEST_COMPLETIONS_PTY_NAME