fix(mailer): Call to custom mail_template_class

[phreaker0]

• Resolves: [Bug]: custom mail template class is broken in 29.0.5 / 28.0.9 #47430

Summary

In case of a custom mail_template_class the class is called with missing parameters (which were introduced in 29.0.5). This patch works for me.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[kesselb]

Thank you very much 👍

[kesselb]

/backport to stable29

[kesselb]

/backport to stable28

[kesselb]

/backport to stable30

[nickvergessen]

Now it breaks real custom templates that do not extend the existing base class, but I guess I missed is_a($class, EMailTemplate::class, true) which means all of them should extend this.

[welcome]

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