dot: improve output to work around GraphViz bug

The related GraphViz issue is
https://gitlab.com/graphviz/graphviz/-/issues/2179

* spot/twaalgos/dot.cc: Avoid initial newline in title.
* NEWS: Mention the bug.
* tests/core/det.test, tests/core/dstar.test,
tests/core/neverclaimread.test, tests/python/automata-io.ipynb: Adjust
test cases.

NEWS

@@ -11,6 +11,9 @@ New in spot 2.10.2.dev  (not yet released)
     32 states due to an optimization of the determinization being
     unintentionally disabled.
 
+  - Work around GraphViz bug #2179 by avoiding unnecessary empty
+    lines in the acceptance conditions shown in dot.
+
 New in spot 2.10.2  (2021-12-03)
 
   Bugs fixed:

spot/twaalgos/dot.cc

@@ -1,5 +1,5 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2011, 2012, 2014-2021 Laboratoire de Recherche
+// Copyright (C) 2011, 2012, 2014-2022 Laboratoire de Recherche
 // et Développement de l'Epita (LRDE).
 // Copyright (C) 2003, 2004  Laboratoire d'Informatique de Paris 6 (LIP6),
 // département Systèmes Répartis Coopératifs (SRC), Université Pierre
@@ -626,7 +626,16 @@ namespace spot
         std::string accstr = aut_->acc().name("d");
         if (accstr.empty())
           return;
-        os_ << nl_ << '[' << accstr << ']';
+        // If a name or an acceptance formula was printed,
+        // we need to add a newline before the human version of the acceptance.
+        //
+        // We used to always add that line, but on unnamed Büchi
+        // automata with double circles, this would (1) create an
+        // empty line, and (2) run into the following GraphViz bug:
+        // https://gitlab.com/graphviz/graphviz/-/issues/2179
+        if (name_ || !dcircles_)
+          os_ << nl_;
+        os_ << '[' << accstr << ']';
       }
 
       void

tests/core/det.test

@@ -1,6 +1,6 @@
 #!/bin/sh
 # -*- coding: utf-8 -*-
-# Copyright (C) 2013-2021 Laboratoire de Recherche et Développement de
+# Copyright (C) 2013-2022 Laboratoire de Recherche et Développement de
 # l'Epita (LRDE).
 #
 # This file is part of Spot, a model checking library.
@@ -113,7 +113,7 @@ run 0 ../ikwiad -x -DC 'GFa & XGFb' > out.tgba
 cat >ex.tgba <<EOF
 digraph "GFa & XGFb" {
   rankdir=LR
-  label="\n[Büchi]"
+  label="[Büchi]"
   labelloc="t"
   node [shape="circle"]
   I [label="", style=invis, width=0]

tests/core/dstar.test

@@ -1,7 +1,7 @@
 #!/bin/sh
 # -*- coding: utf-8 -*-
-# Copyright (C) 2013-2016, 2018, 2020 Laboratoire de Recherche et
+# Copyright (C) 2013-2016, 2018, 2020, 2022 Laboratoire de Recherche
-# Développement de l'Epita (LRDE).
+# et Développement de l'Epita (LRDE).
 #
 # This file is part of Spot, a model checking library.
 #
@@ -85,7 +85,7 @@ run 0 ../ikwiad -d -XDB -R3 dra.dstar | tee stdout
 cat >expected <<EOF
 digraph "" {
   rankdir=LR
-  label="\n[Büchi]"
+  label="[Büchi]"
   labelloc="t"
   node [shape="circle"]
   I [label="", style=invis, width=0]
@@ -132,7 +132,7 @@ run 0 ../ikwiad -d -XDB dsa.dstar | tee stdout
 cat >expected <<EOF
 digraph "" {
   rankdir=LR
-  label="\n[Büchi]"
+  label="[Büchi]"
   labelloc="t"
   node [shape="circle"]
   I [label="", style=invis, width=0]

tests/core/neverclaimread.test

@@ -1,6 +1,6 @@
 #!/bin/sh
 # -*- coding: utf-8 -*-
-# Copyright (C) 2010-2015, 2017-2018, 2020 Laboratoire
+# Copyright (C) 2010-2015, 2017-2018, 2020, 2022 Laboratoire
 # de Recherche et Développement de l'Epita (LRDE).
 #
 # This file is part of Spot, a model checking library.
@@ -134,7 +134,7 @@ run 0 ../ikwiad -XN input > stdout
 cat >expected <<EOF
 digraph "" {
   rankdir=LR
-  label="\n[Büchi]"
+  label="[Büchi]"
   labelloc="t"
   node [shape="circle"]
   I [label="", style=invis, width=0]

tests/python/automata-io.ipynb

@@ -61,7 +61,7 @@
       "\n",
       "digraph \"\" {\n",
       "  rankdir=LR\n",
-      "  label=<<br/>[Büchi]>\n",
+      "  label=<[Büchi]>\n",
       "  labelloc=\"t\"\n",
       "  node [shape=\"circle\"]\n",
       "  node [style=\"filled\", fillcolor=\"#ffffaa\"]\n",