[HUDI-4915][Followup] add UT for the improved avro's serialize/deserialize

[YannByron]

Change Logs

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Impact

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Documentation Update

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• The config description must be updated if new configs are added or the default value of the configs are changed
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  changes to the website.

Contributor's checklist

• Read through contributor's guide
• Change Logs and Impact were stated clearly
• Adequate tests were added if applicable
• CI passed

[YannByron]

@xushiyan @alexeykudinkin follow up #6788

[hudi-bot]

CI report:

• 5f29106 UNKNOWN

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