fix #17258 - use model class only if it is generated (#17490) #1470

Job	Run time
Gradle tests (samples/openapi3/client/extensions/x-auth-id-alias/java/jersey2-java8)	1m 0s
Gradle tests (samples/openapi3/client/petstore/java/jersey2-java8-special-characters)	54s
Gradle tests (samples/openapi3/client/petstore/java/jersey2-java8)	59s
Gradle tests (samples/client/petstore/java/jersey2-java8-localdatetime)	55s
Gradle tests (samples/client/petstore/java/jersey2-java8)	53s
Gradle tests (samples/client/petstore/java/okhttp-gson)	59s
Gradle tests (samples/client/petstore/java/okhttp-gson-group-parameter)	55s
Gradle tests (samples/client/petstore/java/webclient-swagger2)	1m 1s
	7m 36s

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