Fix local_ip handling in KNX options flow

[marvin-w]

Proposed change

THis PR fixes the local_ip handling in the KNX options flow. Previously, it was not able to remove this value again and sometimes saving was also not possible. This PR makes the local_ip required and adds a default of 0.0.0.0 that will behave as if the value is None since the data flow framework can't handle optional values.

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

Additional information

• This PR fixes or closes issue: fixes KNX tunneling ipv4 address throw warning/error #62862, fixes HA integration: Local IP required in ConfigFlow XKNX/xknx#833
• This PR is related to issue:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• For the updated dependencies - a link to the changelog, or at minimum a diff between library versions is added to the PR description.
• Untested files have been added to .coveragerc.

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[probot-home-assistant]

Hey there @Julius2342, @farmio, 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)}}

[farmio]

Tested locally and works! Thanks Marvin!

[farmio]

@marvin-w shouldn't we add this to the 12.8 milestone? Lots of users complaining about this 😬