dotty: fix combination of 's' with 'n'

* src/tgbaalgos/dotty.cc: Add empty label to each cluster
if both 's' and 'n' are used.
* src/tgbatest/neverclaimread.test: Test it.

src/tgbaalgos/dotty.cc

@@ -143,6 +143,10 @@ namespace spot
 	    for (unsigned i = 0; i < sccs; ++i)
 	      {
 		os_ << "  subgraph cluster_" << i << " {\n";
+		if (opt_name_)
+		  // Reset the label, otherwise the graph label would
+		  // be inherited by the cluster.
+		  os_ << "  label=\"\"\n";
 		for (auto s: si->states_of(i))
 		  process_state(s);
 		os_ << "  }\n";

src/tgbatest/neverclaimread.test

@@ -322,17 +322,21 @@ accept_all:
   skip
 }
 EOF
-../../bin/autfilt --dot=sc <input >stdout 2>stderr && exit 1
+../../bin/autfilt --name=%F --dot=nsc <input >stdout 2>stderr && exit 1
 cat >expected <<EOF
 digraph G {
   rankdir=LR
+  label="-"
+  labelloc="t"
   node [shape="circle"]
   I [label="", style=invis, width=0]
   I -> 0
   subgraph cluster_0 {
+  label=""
   1 [label="1", peripheries=2]
   }
   subgraph cluster_1 {
+  label=""
   0 [label="0"]
   }
   0 -> 1 [label="b"]