Update javadoc for EMPTY_BLOB_ID

Found original version with:

  $ git describe --contains $(git blame fc4d4b5340 org.eclipse.jgit/src/org/eclipse/jgit/lib/Constants.java | grep EMPTY_BLOB_ID | cut -d' ' -f1)
  v0.9.1~164

Change-Id: I1f86bde42615c8b64064894bde44f82660b38d0f

org.eclipse.jgit/src/org/eclipse/jgit/lib/Constants.java

@@ -687,7 +687,11 @@ public final class Constants {
 	 */
 	public static final String COMMIT_EDITMSG = "COMMIT_EDITMSG";
 
-	/** objectid for the empty blob */
+	/**
+	 * Well-known object ID for the empty blob.
+	 *
+	 * @since 0.9.1
+	 */
 	public static final ObjectId EMPTY_BLOB_ID = ObjectId
 			.fromString("e69de29bb2d1d6434b8b29ae775ad8c2e48c5391");