fix: avoid generic NPE when refering to missing schema

Created by: tomasbjerre

I was getting a generic NPE that I was unable to analyze:

...
        at org.gradle.internal.concurrent.ManagedExecutorImpl$1.run(ManagedExecutorImpl.java:48)
        at org.gradle.internal.concurrent.ThreadFactoryImpl$ManagedThreadRunnable.run(ThreadFactoryImpl.java:56)
Caused by: java.lang.NullPointerException
        at org.openapitools.codegen.DefaultCodegen.updateAllModels(DefaultCodegen.java:568)
        at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:497)
        at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:888)
        at org.openapitools.generator.gradle.plugin.tasks.GenerateTask.doWork(GenerateTask.kt:716)
        ... 223 more
...

Here: https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java#L568

This change gives me something to investigate:

...
        at org.gradle.internal.concurrent.ThreadFactoryImpl$ManagedThreadRunnable.run(ThreadFactoryImpl.java:56)
Caused by: java.lang.NullPointerException: _object in {account=class ComposedSchema {
    class Schema {
...