feat(mdns): support null value for boolean txt records

[tanyanquan]

Description

This PR adds support for NULL values to be used in mdns txt records in order to differentiate "myKey" and "myKey=" txt records.

These two records are considered to be different in RFC 6763 Section 6.4, so it is important for us to have a way to handle them differently.

Previous usage (if NULL is not supported):
"myKey" -> key = "myKey", value = ""; "myKey=" -> key = "myKey", value = ""

Suggested changes:
"myKey" -> key = "myKey", value = NULL; "myKey=" -> key = "myKey", value = ""

Related

Testing

• https://docs.espressif.com/projects/esp-thread-br/en/latest/codelab/service_discovery.html
  • Use different txt records such as 0778797a3d58595a (xyz=XYZ), 0778797a6158595a (xyzaXYZ), 0778797a6158593d (xyzaXY=)

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Checklist

Before submitting a Pull Request, please ensure the following:

• 🚨 This PR does not introduce breaking changes.
• All CI checks (GH Actions) pass.
• Documentation is updated as needed.
• Tests are updated or added as necessary.
• Code is well-commented, especially in complex areas.
• Git history is clean — commits are squashed to the minimum necessary.