Fix nullness of currentRecordId and currentRecordOffset

Author: kennknowles

runners/core-java/src/main/java/org/apache/beam/runners/core/OutputAndTimeBoundedSplittableProcessElementInvoker.java

@@ -387,12 +387,12 @@ public PaneInfo pane() {
     }
 
     @Override
-    public String currentRecordId() {
+    public @Nullable String currentRecordId() {
       return element.getRecordId();
     }
 
     @Override
-    public Long currentRecordOffset() {
+    public @Nullable Long currentRecordOffset() {
       return element.getRecordOffset();
     }
 

runners/core-java/src/main/java/org/apache/beam/runners/core/SimpleDoFnRunner.java

@@ -470,12 +470,12 @@ public Instant timestamp() {
     }
 
     @Override
-    public String currentRecordId() {
+    public @Nullable String currentRecordId() {
       return elem.getRecordId();
     }
 
     @Override
-    public Long currentRecordOffset() {
+    public @Nullable Long currentRecordOffset() {
       return elem.getRecordOffset();
     }
 

sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/DoFn.java

@@ -326,10 +326,10 @@ public abstract class ProcessContext extends WindowedContext {
     public abstract PaneInfo pane();
 
     @Pure
-    public abstract String currentRecordId();
+    public abstract @Nullable String currentRecordId();
 
     @Pure
-    public abstract Long currentRecordOffset();
+    public abstract @Nullable Long currentRecordOffset();
   }
 
   /** Information accessible when running a {@link DoFn.OnTimer} method. */