An Email with only emoji in the subject line is displaying "Subject is empty" error and is not approving, or emoji fail to render elsewhere in the email.
If the subject line only consists of emoji, consider adding text.
The Email Editor was not designed to insert emoji. However some have been able to insert emoji from outside sources. Those with the most success are inserting UTF-8 encoded emoticons. This specific encoded emoticon can be inserted (copy/paste) through a 3rd party website, located with a general web search: "UTF-8 emoticons/emoji".
If the inserted emoji has a different encoding, it may fail to render in the Email Editor, and it may fail to render upon delivery as well.
Rendering upon delivery will be reliant on the recipient server. If the emoji is considered invalid, then the 'diamond with question mark' character may remain.
Alternatively, basic symbols could be used instead of emoji, as they are ASCII characters and not reliant on extra encoding. 3rd party sites, identified with a websearch: "ASCII symbols", can be a resource.
Another option can be q-encoding the emoji in UTF-8 format to render a string of code that can be used. This code tells the email client to render the desired emoji. To q-encode, copy and paste the desired emoji through a Unicode to UTF-8 translation tool, such as https://tools.bluestatedigital.com/kb/subject-line-assistant and then insert the translated code.