Fix: enter SmtpHook before trying to read properties

[m-rossi]

closes: #49920

This PR tries to fix the issue #49920, the SmtpHook is tried to be used and its properties are only available once the .conn attibute is available, which is only the case in the with statement.

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[boring-cyborg]

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[shahar1]

Welcome to Apache Airflow and thanks for your contribution!
I hope you could figure out #49920 as well :)

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.