fix(registry): handle empty targets in TXT records logging an error

[saikatharryc]

Description

Fixes #5129

Context: TXT record with routing policy on GCP has no direct single target, as a result, the direct index access to first element fails. (Such routing policy based records not necessarily be part of the external dns management area, it could be created or updated manually that can cause this issue.)

How to test manually:
Let the external-dns run as is , the google DNS zone you are using , create a TXT record with weighted routing policy.
Current version should crash with the error in the issue.
in the PR changes will make sure it gets passed by, with a error line like below

ERRO[0002] TXT record <DNS> has no targets

Checklist

• Unit tests updated
• End user documentation updated

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• ❌ - login: @saikatharryc / name: Saikat Chakrabortty . The commit (b060ce4) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please submit a support request ticket.

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