[fix] Ollama - Drop model registry check & improve description

[paxcema]

Description

• create_validation(): dropped model registry check, as the API stopped returning officially supported models
• describe(): now robust against missing information in the model's description

Type of change

• 🐛 Bug fix (non-breaking change which fixes an issue)

Verification Process

To ensure the changes are working as expected:

• Test Location: Specify the URL or path for testing.
• Verification Steps: Outline the steps or queries needed to validate the change. Include any data, configurations, or actions required to reproduce or see the new functionality.

Additional Media:

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Checklist:

• My code follows the style guidelines(PEP 8) of MindsDB.
• I have appropriately commented on my code, especially in complex areas.
• Necessary documentation updates are either made or tracked in issues.
• Relevant unit and integration tests are updated or added.