[java] fix ClassCastException in toDefaultValue()

[jmini]

Fixes #3761

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
• Filed the PR against the correct branch: master, 4.1.x, 5.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language. java: @bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04)

Description of the PR

This PR fixes the java.lang.ClassCastException:

java.lang.ClassCastException: io.swagger.v3.oas.models.media.ObjectSchema cannot be cast to io.swagger.v3.oas.models.media.ArraySchema
	at org.openapitools.codegen.languages.AbstractJavaCodegen.toDefaultValue(AbstractJavaCodegen.java:721)
	at org.openapitools.codegen.DefaultCodegen.fromProperty(DefaultCodegen.java:2058)
	at org.openapitools.codegen.DefaultCodegen.fromProperty(DefaultCodegen.java:2271)
	at org.openapitools.codegen.DefaultCodegen.addVars(DefaultCodegen.java:3706)
	at org.openapitools.codegen.DefaultCodegen.addVars(DefaultCodegen.java:3654)
	at org.openapitools.codegen.DefaultCodegen.fromModel(DefaultCodegen.java:1918)
	at org.openapitools.codegen.languages.AbstractJavaCodegen.fromModel(AbstractJavaCodegen.java:942)
	at org.openapitools.codegen.DefaultGenerator.processModels(DefaultGenerator.java:1198)
	at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:465)

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