[php] bugfix: Array and Map inner schema definition can be missing.

[DenKoren]

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Description of the PR

/cc @jebentier @dkarlovi @mandrean @jfastnacht @ackintosh @ybelenko

Bugfix is the same to #1363 but for PHP code generator.

This bug appeared during mattermost v4 api code generation (at least for the master version. At the moment of test it was 87a2f9b014f07e4431aa5e884029a5bb4e9f0b68)

The error produced by missing schema definition is listed below.

[main] WARN o.o.codegen.utils.ModelUtils - Multiple schemas found, returning only the first one
Exception in thread "main" java.lang.RuntimeException: Could not process model 'inline_object_43'.Please make sure that your schema is correct!
at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:434)
at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:865)
at org.openapitools.codegen.cmd.Generate.run(Generate.java:349)
at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:62)
Caused by: java.lang.NullPointerException
at org.openapitools.codegen.utils.ModelUtils.isArraySchema(ModelUtils.java:339)
at org.openapitools.codegen.languages.AbstractPhpCodegen.getTypeDeclaration(AbstractPhpCodegen.java:345)
at org.openapitools.codegen.languages.AbstractPhpCodegen.getTypeDeclaration(AbstractPhpCodegen.java:348)
at org.openapitools.codegen.DefaultCodegen.fromProperty(DefaultCodegen.java:1966)
at org.openapitools.codegen.DefaultCodegen.addVars(DefaultCodegen.java:3391)
at org.openapitools.codegen.DefaultCodegen.addVars(DefaultCodegen.java:3362)
at org.openapitools.codegen.DefaultCodegen.addVars(DefaultCodegen.java:3349)
at org.openapitools.codegen.DefaultCodegen.fromModel(DefaultCodegen.java:1675)
at org.openapitools.codegen.DefaultGenerator.processModels(DefaultGenerator.java:1103)
at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:429)

[ackintosh]

Thanks for this PR, which looks good to me.

[ackintosh]

By the way, in OpenAPI-Specification, an inner schema must be defined. So I think root of problem is mattermost-openapi-v4.yaml and swagger-parser.