termux-notification: handle multiline content

fixes termux/termux-api#274

Author: xalexalex

scripts/termux-notification

@@ -141,5 +141,4 @@ if [ -n "$OPT_TITLE" ]; then set -- "$@" --es title "$OPT_TITLE"; fi
 if [ -n "$OPT_TYPE" ]; then set -- "$@" --es type "$OPT_TYPE"; fi
 if [ -n "$OPT_VIBRATE" ]; then set -- "$@" --ela vibrate "$OPT_VIBRATE"; fi
 
-# Note that we want to accept an empty content (--content "").
-echo ${OPT_CONTENT:=""} | /data/data/com.termux/files/usr/libexec/termux-api Notification "$@"
+echo "$OPT_CONTENT" | /data/data/com.termux/files/usr/libexec/termux-api Notification "$@"