Fix a bug in spot.complete()

spot.complete() could complete an empty co-Büchi automaton into an
automaton accepting everything.

* NEWS: Document it
* spot/twaalgos/complete.cc: Fix it
* tests/core/complete.test, tests/core/prodor.test: Test it

tests/core/complete.test

@@ -25,6 +25,14 @@ set -e
 
 cat >automaton <<EOF
 HOA: v1
+States: 1
+Start: 0
+AP: 1 "a"
+Acceptance: 1 Fin(0)
+--BODY--
+State: 0
+--END--
+HOA: v1
 States: 2
 Start: 0
 AP: 1 "a"
@@ -107,6 +115,18 @@ EOF
 
 cat >expected <<EOF
 HOA: v1
+States: 1
+Start: 0
+AP: 1 "a"
+acc-name: co-Buchi
+Acceptance: 1 Fin(0)
+properties: trans-labels explicit-labels state-acc colored complete
+properties: deterministic
+--BODY--
+State: 0 {0}
+[t] 0
+--END--
+HOA: v1
 States: 3
 Start: 0
 AP: 1 "a"
@@ -210,12 +230,12 @@ Start: 0&1
 AP: 0
 acc-name: co-Buchi
 Acceptance: 1 Fin(0)
-properties: trans-labels explicit-labels state-acc complete
+properties: trans-labels explicit-labels state-acc colored complete
 properties: deterministic univ-branch
 --BODY--
-State: 0
+State: 0 {0}
 [t] 0
-State: 1
+State: 1 {0}
 [t] 0
 --END--
 EOF

tests/core/prodor.test

@@ -24,6 +24,32 @@ set -e
 ltl2tgba=ltl2tgba
 autfilt=autfilt
 
+# Two empty automata
+cat >a1.hoa <<EOF
+HOA: v1
+States: 1
+Start: 0
+AP: 1 "a"
+Acceptance: 1 Fin(0)
+--BODY--
+State: 0
+--END--
+EOF
+cat >a2.hoa <<EOF
+HOA: v1
+States: 1
+Start: 0
+AP: 1 "a"
+Acceptance: 0 f
+--BODY--
+State: 0
+[t] 0
+--END--
+EOF
+
+# the OR product of two empty automata should be empty
+$autfilt --product-or a1.hoa a2.hoa -H | $autfilt --is-empty
+
 $ltl2tgba -DH 'GFa' > gfa.hoa
 $ltl2tgba -DH 'FGb' > fgb.hoa
 $autfilt --product-or gfa.hoa fgb.hoa -H > por.hoa