Check and disallow a relative path for sqlite

[yenchenLiu]

related: #22014

Since flask-session was added in version 2.2.4, Flask needs to connect to the database.
However, when establishing a database connection, if a relative path is used, DB path will not be correctly used. So when building a Flask app, we check if the database path is a relative path, and if so, an error message will be displayed.

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[boring-cyborg]

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[potiuk]

This is really cool find @yenchenLiu - thanks for finding out the root cause ! It was a difficult one!

[boring-cyborg]

Awesome work, congrats on your first merged pull request!