[gnome-tetravex] Do not add empty tiles inversion to history.

[Arnaud B. <arnaudb src gnome org>]

commit f801b18acb1de505a948eb1012d50c0a1103097f
Author: Arnaud Bonatti <arnaud bonatti gmail com>
Date:   Sun Sep 22 05:16:20 2019 +0200

    Do not add empty tiles inversion to history.

 src/puzzle.vala | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/src/puzzle.vala b/src/puzzle.vala
index a240c35..02bca0d 100644
--- a/src/puzzle.vala
+++ b/src/puzzle.vala
@@ -248,6 +248,9 @@ private class Puzzle : Object
 
         Tile? t0 = board [x0, y0];
         Tile? t1 = board [x1, y1];
+        if (t0 == null && t1 == null)   // might happen when move_up and friends are called
+            return;
+
         board [x0, y0] = t1;
         board [x1, y1] = t0;