fix(main): fix ImportError on examples dags

[xionams]

this pull request is related to: apache/airflow-site#945
closes: apache/airflow-site#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]

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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]

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