fix(composer): handle missing message body gracefully

[kesselb]

For #12320

How to test

Index: lib/Controller/MessagesController.php
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
diff --git a/lib/Controller/MessagesController.php b/lib/Controller/MessagesController.php
--- a/lib/Controller/MessagesController.php	(revision 45cdf0a8e9e5c08e69454c207d8845b86ca59223)
+++ b/lib/Controller/MessagesController.php	(date 1769552067333)
@@ -574,7 +574,7 @@
 	public function getHtmlBody(int $id, bool $plain = false): Response {
 		try {
 			try {
-				$message = $this->mailManager->getMessage($this->currentUserId, $id);
+				$message = $this->mailManager->getMessage($this->currentUserId, PHP_INT_MIN);
 				$mailbox = $this->mailManager->getMailbox($this->currentUserId, $message->getMailboxId());
 				$account = $this->accountService->find($this->currentUserId, $mailbox->getAccountId());
 			} catch (DoesNotExistException) {

Render template without skeleton

• Open Inbox
• Pick any message with an html body

B	A

I'm aware the font looks off, and no, I'm not going to fix that 🤣

Handle missing body

• Open Drafts
• Click on an existing draft

B	A

[kesselb]

/backport to stable5.7

[kesselb]

/backport to stable5.6