[BUGFIX] Avoid repeated e-mail content generation in FluidEmail

Body content generation may be triggered by calling some getter or
indirectly when the mail is sent (via `ensureValidity`).

This changes ensures that multiple tries to generate the body are only
executed if the state of the view has actually changed.

This enables extensions to manipulate the content of an e-mail via
events without getting their changes overridden again when the mail is
sent.

Resolves: #99944
Releases: main, 11.5
Change-Id: I5eced5710feeb66e6f1e32f10f710d388bc43a62
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78251


Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: core-ci <typo3@b13.com>