BlameGenerator: Annotate #getRenameDetector as Nullable

The renameDetector member returned by this method will be null when
following file renames has been disabled by previously calling:

  setFollowFileRenames(false).

Annotate it as @Nullable and update the Javadoc to explicitly
document the null return.

Change-Id: I9bdf443a64cf3c45352d3ab023051a2e11f7426d
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

org.eclipse.jgit/src/org/eclipse/jgit/blame/BlameGenerator.java

@@ -51,6 +51,7 @@ import java.io.IOException;
 import java.util.Collection;
 import java.util.Collections;
 
+import org.eclipse.jgit.annotations.Nullable;
 import org.eclipse.jgit.blame.Candidate.BlobCandidate;
 import org.eclipse.jgit.blame.Candidate.ReverseCandidate;
 import org.eclipse.jgit.blame.ReverseWalk.ReverseCommit;
@@ -238,11 +239,13 @@ public class BlameGenerator implements AutoCloseable {
 	}
 
 	/**
-	 * Obtain the RenameDetector if {@code setFollowFileRenames(true)}.
+	 * Obtain the RenameDetector, allowing the application to configure its
+	 * settings for rename score and breaking behavior.
 	 *
-	 * @return the rename detector, allowing the application to configure its
-	 *         settings for rename score and breaking behavior.
+	 * @return the rename detector, or {@code null} if
+	 *         {@code setFollowFileRenames(false)}.
 	 */
+	@Nullable
 	public RenameDetector getRenameDetector() {
 		return renameDetector;
 	}