ATLAS-5199: Impala "startTime" attribute changes to null from EMPTY string, after 1 retry on NotificationHookConsumer

[aditya-gupta36]

What changes were proposed in this pull request?

1. Fixed attribute normalization in AtlasStructType: Added logic to preserve non-null original values when normalization returns null, preventing empty strings in date fields from being converted to null during retry attempts (4 methods: normalizeAttributeValues, normalizeAttributeValuesForUpdate for both AtlasStruct and Map variants)
2. Added empty string handling in EntityGraphMapper: to transform empty strings to appropriate default values (e.g., Date(0) for date types) before storing to JanusGraph, preventing schema violation exceptions
3. Resolved retry failure for NotificationHookConsumer: Fixed the issue where Kafka messages with empty date field values would fail validation on retry with "mandatory attribute value missing" error after normalization converted empty strings to null

How was this patch tested?

1. Tested on docker by prodcuing kafka messages and consuming it wrt kafka-console-consumer.sh
2. mvn build