zsh:keys:cmd-on-enter: Check PREBUFFER as well

When typing a multi-line input (e.g. `echo foo\<CR>`) BUFFER can be
empty, even though the entire input is not.

Fix this by checking if PREBUFFER and BUFFER are empty.

.config/zsh/zshrc.d/60-keys.zsh

@@ -177,7 +177,7 @@ bindkey '^[[46;5u' default_dot
 CMDS_ON_ENTER=(ll gs)
 REQUIREMENTS_CMDS_ON_ENTER=(true "git rev-parse")
 function cmd-on-enter {
-	if [[ -z $BUFFER ]]; then
+	if [[ -z "${PREBUFFER}${BUFFER}" ]]; then
 		# Overwrite BUFFER and default to ll
 		BUFFER=" ${CMDS_ON_ENTER[${cmd_on_enter_idx:=1}]}"