hoa: do not output empty highlighting maps

* spot/twaalgos/hoa.cc: Here.
* tests/python/parsetgba.py: Test it.

tests/python/parsetgba.py

@@ -123,3 +123,26 @@ State: 4 {0}
 [1] 4
 [2] 4
 --END--""")
+a3.highlight_edges([5, 6], None)
+tc.assertEqual(a3.to_str("hoa", "1.1"), """HOA: v1.1
+States: 5
+Start: 0
+AP: 3 "a" "b" "c"
+acc-name: Buchi
+Acceptance: 1 Inf(0)
+properties: trans-labels explicit-labels state-acc !complete
+properties: !deterministic exist-branch
+--BODY--
+State: 0
+[0] 1
+[0] 2
+State: 1
+[0] 3
+State: 2
+[0] 4
+State: 3 {0}
+[0] 3
+State: 4 {0}
+[1] 4
+[2] 4
+--END--""")