Bugfix/fix npe open api generator

[sebastian-mrozek]

Caused by: java.lang.NullPointerException
        at org.jetbrains.jps.javac.APIWrappers$FilerWrapper.createResource(APIWrappers.java:156)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:566)
        at org.jetbrains.jps.javac.APIWrappers$1.invoke(APIWrappers.java:213)
        at com.sun.proxy.$Proxy27.createResource(Unknown Source)
        at io.avaje.http.generator.core.openapi.DocContext.createMetaWriter(DocContext.java:203)
        at io.avaje.http.generator.core.openapi.DocContext.writeApi(DocContext.java:180)
        at io.avaje.http.generator.core.BaseProcessor.writeOpenAPI(BaseProcessor.java:85)
        at io.avaje.http.generator.core.BaseProcessor.process(BaseProcessor.java:60)
        at org.jetbrains.jps.javac.APIWrappers$ProcessorWrapper.process(APIWrappers.java:109)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:566)
        at org.jetbrains.jps.javac.APIWrappers$1.invoke(APIWrappers.java:213)
        at com.sun.proxy.$Proxy25.process(Unknown Source)
        at jdk.compiler/com.sun.tools.javac.processing.JavacProcessingEnvironment.callProcessor(JavacProcessingEnvironment.java:980)
        at jdk.compiler/com.sun.tools.javac.processing.JavacProcessingEnvironment$DiscoveredProcessors$ProcessorStateIterator.runContributingProcs(JavacProcessingEnvironment.java:816)
        at jdk.compiler/com.sun.tools.javac.processing.JavacProcessingEnvironment$Round.run(JavacProcessingEnvironment.java:1220)
        at jdk.compiler/com.sun.tools.javac.processing.JavacProcessingEnvironment.doProcessing(JavacProcessingEnvironment.java:1356)
        at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.processAnnotations(JavaCompiler.java:1258)
        at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:936)
        at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.lambda$doCall$0(JavacTaskImpl.java:104)
        at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.handleExceptions(JavacTaskImpl.java:147)