AVRO-4321: Fix NPE on getConversionByClass

[RyanSkraba]

What is the purpose of the change

Fix the NPE caused by AVRO-3989 on classes that have a datum with a Conversion but not a LogicalType.

Verifying this change

This change added tests getConversionByClassWithNullLogicalTypeReturnsNull

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

[Copilot]

Pull request overview

Fixes a NullPointerException in the Java Avro data model when callers ask for a conversion by datum class but provide a null LogicalType, which can happen for classes that have conversions registered by default (e.g., BigDecimal) even when no logical type is available.

Changes:

• Add a null guard in GenericData#getConversionByClass(Class, LogicalType) to return null instead of dereferencing logicalType.
• Add a JUnit test covering the null logical type case via ReflectData.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lang/java/avro/src/main/java/org/apache/avro/generic/GenericData.java	Prevents NPE by returning null when logicalType is null in conversion lookup.
lang/java/avro/src/test/java/org/apache/avro/reflect/TestReflectData.java	Adds regression test ensuring getConversionByClass(..., null) returns null (no NPE).

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[RyanSkraba]

Cherry-picked to branch-1.12.