log: Display "(no subject)" for commits without a subject

This resolves the ambiguity for a message with a body but no subject,
and also hints that OSTree is using two bytes to store the empty
commit subject/body (when instead they could be stored, or rather,
not stored, as part of the metadata)

Closes: #305
Approved by: cgwalters

src/ostree/ot-dump.c

@@ -118,8 +118,15 @@ dump_commit (GVariant            *variant,
       g_print ("Version: %s\n", version);
     }
 
-  g_print ("\n");
-  dump_indented_lines (subject);
+  if (subject[0])
+    {
+      g_print ("\n");
+      dump_indented_lines (subject);
+    }
+  else
+    {
+      g_print ("(no subject)\n");
+    }
 
   if (body[0])
     {