How to send an invisible/blank/empty Signal message?

The Signal messenger app does not allow you to send a invisible/blank/empty message using regular spaces. If you just enter a "space" using and try to send the message, Signal will refuse with a "Message is empty!" error message. To work around this problem, you have to use a different character that looks like a space, but is not recognized as a space by the Signal app. Such a character is the Unicode character U+3164 HANGUL FILLER, which you can copy to your clipboard using the following "Copy" button:

After copying the character and pasting it into your Signal message, you can successfully send the message!