make EventProcessor.process Nullable

Null return is used when the event should be dropped. This aligns with
the current handling in code and also with the Sentry unified API
definition of event processors.

Author: davemt

sentry-core/src/main/java/io/sentry/core/EventProcessor.java

@@ -3,5 +3,6 @@
 import org.jetbrains.annotations.Nullable;
 
 public interface EventProcessor {
+  @Nullable
   SentryEvent process(SentryEvent event, @Nullable Object hint);
 }

sentry-core/src/main/java/io/sentry/core/MainEventProcessor.java

@@ -6,6 +6,7 @@
 import java.util.ArrayList;
 import java.util.List;
 import org.jetbrains.annotations.ApiStatus;
+import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
 @ApiStatus.Internal
@@ -38,7 +39,7 @@ public final class MainEventProcessor implements EventProcessor {
   }
 
   @Override
-  public SentryEvent process(SentryEvent event, @Nullable Object hint) {
+  public @NotNull SentryEvent process(SentryEvent event, @Nullable Object hint) {
     if (event.getPlatform() == null) {
       // this actually means JVM related.
       event.setPlatform("java");