Template: fix "Failed to delete buffer" error

[JanTvrdik]

• bug fix: yes
• BC break: no

This fixes error

ob_end_clean(): Failed to delete buffer. No buffer to delete

but I'm not sure if this is the best way to do it.

It happens with combination of

• using Engine::renderToString()
• having configured exception handler $latte->setExceptionHandler(fn(Throwable $e) => throw $e);
• error happends inside {try}...{/try}

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Latte 3.0.2, standalone usage