I love this kind of question, it's like a guessing game and makes me feel like a teenager :)
Without additional context, it can be difficult to determine the exact meaning behind a person using the heart emoji when saying goodnight. Here are some possible explanations:
Friendly gesture: The person might just be sending the heart emoji as a friendly gesture to express his positive feelings towards you. In this case, the heart emoji probably doesn't have any romantic meaning.
Romantic interest: This guy might use the heart emoji to show his romantic interest in you. If he keeps sending heart emojis and flirting messages, it could be a sign that he's interested in a romantic relationship with you.
Personal communication style: Some people regularly use heart emojis and other symbols in text messages as part of their personal communication style. In this case, the heart emoji doesn't necessarily indicate any deeper meaning. It could be that he wants you to take his heart/love.
The point is - if you have to go to a website and ask someone what a heart emoji means, I guess the two of you probably don't have feelings, I guess?
If you're a teenager, it's really okay to guess over and over again, but if you're over 20 and not sure what other people are trying to tell you, just ask them, don't guess :)
Good luck!