fix(main): fix ImportError on examples dags

[xionams]

this pull request is related to: https://github.com/apache/airflow-site/issues/945

the solution of "skipped" on the dag-loading when the virtualenv is not installed is wrap the dag loading with try-except and return None either using this logic will guarantee the ImportError exception will not stored in the DB and not thrown on the homepage as an error. so if virtualenv is not installed the examples are not loaded and the exception will not thrown

tested on Ubuntu WSL2

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[boring-cyborg[bot]]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contributors' Guide (https://github.com/apache/airflow/blob/main/contributing-docs/README.rst) Here are some useful points:

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[potiuk]

Just one static check to fix

[eladkal]

i submitted fixes for the static checks. It should pass now

[boring-cyborg[bot]]

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