Fix CreateMessageRequest not being able to send text messages to channel

[surajkumar]

The following command was failing:

guild.channel()
        .createMessage(new CreateMessageBuilder(message.channelId()).content("Hello World!"))
        .onError(System.err::println)
        .onSuccess(System.out::println);

With the error: {"message": "Cannot send an empty message", "code": 50006}

This was happening because we were trying to send a multi part body for regular text messages. The sending of files needs further testing however for now I have fixed our ability to send regular text messages to channels.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

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