improve messaging when no nets/opens/shorts are detected

snarled/trace.py

@@ -68,16 +68,22 @@ class TraceAnalysis:
         ss += '\n(groups of electrically connected labels)\n'
         for net in sort_nets(self.nets):
             ss += '\t' + format_net(net) + '\n'
+        if not self.nets:
+            ss += '\t<NO NETS FOUND>'
 
         ss += '\nOpens'
         ss += '\n(2+ nets containing the same name)\n'
         for label, count in sorted(self.opens.items()):
             ss += f'\t{label} : {count} nets\n'
+        if not self.opens:
+            ss += '\t<No opens found>'
 
         ss += '\nShorts'
         ss += '\n(2+ unique names for the same net)\n'
         for net in sort_nets(self.shorts):
             ss += '\t' + format_net(net) + '\n'
+        if not self.shorts:
+            ss += '\t<No shorts found>'
 
         ss += '=============\n'
         return ss