bluetooth: samples: mtu_update: peripheral: Add NULL check

[sayooj-atmosic]

Add NULL check for connection context, and only send the notification if connection is established.

[github-actions]

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[jhedberg]

Looks fine, although the sample seems pretty naively implemented. It could be further improved by using e.g. a semaphore to not do any looping until it's actually connected.

[Thalley]

Change is OK, although I think it makes more sense to not print anything when not connected

[github-actions]

Hi @sayooj-atmosic!
Congratulations on getting your very first Zephyr pull request merged 🎉🥳. This is a fantastic achievement, and we're thrilled to have you as part of our community!

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Thank you for your valuable input, and we look forward to seeing more of your contributions in the future! 🪁