@Path("/")
public class OptionalConfigPropertyInjectionResource {
 
    public static final String OPTIONAL_PROPERTY_NAME = "org.jboss.resteasy.test.optionalProperty";   

    @Inject
    @ConfigProperty(name = OPTIONAL_PROPERTY_NAME)
    private Optional<String> optionalProperty;   

    @GET
    @Produces("text/plain")
     public String getOptionalProperty(){
         return optionalProperty.orElse(null);
     }
}

Caused by: java.lang.NullPointerException
	at org.jboss.resteasy.core.InjectorFactoryImpl$OptionalInjections$DelegatingInjector.inject(InjectorFactoryImpl.java:335)
	at org.jboss.resteasy.core.PropertyInjectorImpl.inject(PropertyInjectorImpl.java:145)
	at org.jboss.resteasy.cdi.JaxrsInjectionTarget.inject(JaxrsInjectionTarget.java:74)
	at org.jboss.weld.bean.ManagedBean.create(ManagedBean.java:161)
	at org.jboss.weld.contexts.AbstractContext.get(AbstractContext.java:96)
	at org.jboss.weld.bean.ContextualInstanceStrategy$DefaultContextualInstanceStrategy.get(ContextualInstanceStrategy.java:100)
	at org.jboss.weld.bean.ContextualInstanceStrategy$CachingContextualInstanceStrategy.get(ContextualInstanceStrategy.java:177)
	at org.jboss.weld.bean.ContextualInstance.get(ContextualInstance.java:50)
	at org.jboss.weld.bean.proxy.ContextBeanInstance.getInstance(ContextBeanInstance.java:102)
	at org.jboss.weld.bean.proxy.ProxyMethodHandler.getInstance(ProxyMethodHandler.java:131)
	at org.jboss.resteasy.test.microprofile.config.resource.OptionalConfigPropertyInjectionResource$Proxy$_$$_WeldClientProxy.getOptionalProperty(Unknown Source)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)