fix(avro-schema): fix NPE for null enum values

[joscha]

Fixes:

Download 7.9.0-20241003.092246-97 ...
Downloaded 7.9.0-20241003.092246-97
[[avro-schema] spec.json] [main] INFO  o.o.codegen.DefaultGenerator - Generating with dryRun=false
[[avro-schema] spec.json] [main] INFO  o.o.codegen.DefaultGenerator - OpenAPI Generator: avro-schema (schema)
[[avro-schema] spec.json] [main] INFO  o.o.codegen.DefaultGenerator - Generator 'avro-schema' is considered beta.
[[avro-schema] spec.json] Exception in thread "main" java.lang.RuntimeException: Could not process model 'Field'.Please make sure that your schema is correct!
[[avro-schema] spec.json] 	at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:528)
[[avro-schema] spec.json] 	at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:453)
[[avro-schema] spec.json] 	at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:1303)
[[avro-schema] spec.json] 	at org.openapitools.codegen.cmd.Generate.execute(Generate.java:535)
[[avro-schema] spec.json] 	at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
[[avro-schema] spec.json] 	at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)
[[avro-schema] spec.json] Caused by: java.lang.NullPointerException
[[avro-schema] spec.json] 	at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:197)
[[avro-schema] spec.json] 	at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1708)
[[avro-schema] spec.json] 	at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:509)
[[avro-schema] spec.json] 	at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499)
[[avro-schema] spec.json] 	at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:921)
[[avro-schema] spec.json] 	at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
[[avro-schema] spec.json] 	at java.base/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:682)
[[avro-schema] spec.json] 	at org.openapitools.codegen.languages.AvroSchemaCodegen.buildEnumVars(AvroSchemaCodegen.java:203)
[[avro-schema] spec.json] 	at org.openapitools.codegen.DefaultCodegen.updateCodegenPropertyEnum(DefaultCodegen.java:6604)
[[avro-schema] spec.json] 	at org.openapitools.codegen.DefaultCodegen.postProcessModelsEnum(DefaultCodegen.java:792)
[[avro-schema] spec.json] 	at org.openapitools.codegen.languages.AvroSchemaCodegen.postProcessModels(AvroSchemaCodegen.java:177)
[[avro-schema] spec.json] 	at org.openapitools.codegen.DefaultGenerator.processModels(DefaultGenerator.java:1791)
[[avro-schema] spec.json] 	at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:523)
[[avro-schema] spec.json] 	... 5 more

bug was introduced in #19549

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[joscha]

this is the meaningful change of this PR