Fix KNX onboarding when there is no yaml config defined yet

[farmio]

Proposed change

Fix KNX onboarding when there is no yaml config defined yet.

Previously the integration setup would just fail after finishing config_flow when there is no knx: key in configuration.yaml yet. This caused some confusion.
Now a warning is logged and the integration is useable (eg. services like knx.send).
We can't really distinguish between no yaml config and a reload at this point.

There are also some code movements in config_flow.py in this PR which I thought are really not worth a separate PR.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

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• This PR fixes or closes issue: fixes #
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[probot-home-assistant]

Hey there @Julius2342, @marvin-w, mind taking a look at this pull request as it has been labeled with an integration (knx) you are listed as a code owner for? Thanks!
_{^{(message by CodeOwnersMention)}}

[balloob]

Should this go into a hot fix?