reftable: clarify that LogCursor may return a null ReflogEntry

Change-Id: I1a4d5c262cd196dca37876aec00bb974a45e9fcd
Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/LogCursor.java

@@ -45,6 +45,7 @@ package org.eclipse.jgit.internal.storage.reftable;
 
 import java.io.IOException;
 
+import org.eclipse.jgit.annotations.Nullable;
 import org.eclipse.jgit.lib.ReflogEntry;
 
 /**
@@ -78,8 +79,9 @@ public abstract class LogCursor implements AutoCloseable {
 	/**
 	 * Get current log entry.
 	 *
-	 * @return current log entry.
+	 * @return current log entry. Maybe null if we are producing deletions.
 	 */
+	@Nullable
 	public abstract ReflogEntry getReflogEntry();
 
 	/** {@inheritDoc} */