visualizer: suppress interface netmask length when address is unspecified

The InterfaceTableVisualizer default format "%N %\%a%/%l" guards the
slash directive (%/) on the network address being specified, but %l
printed the IPv4 netmask length unconditionally. Any interface with an
unspecified address carries the all-ones default netmask, so its label
rendered as "<unspec>32" -- an orphaned length with no real subnet
behind it.

Guard %l the same way %/ is guarded: return "" when the IPv4 address
is unspecified. Labels now read "<unspec>" instead of "<unspec>32",
and "192.168.0.10/24" once an address is assigned.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: adamgeorge309

src/inet/visualizer/base/InterfaceTableVisualizerBase.cc

@@ -42,7 +42,7 @@ std::string InterfaceTableVisualizerBase::DirectiveResolver::resolveDirective(ch
         case 'l': // TODO Ipv4 or Ipv6
 #ifdef INET_WITH_IPv4
             if (auto ipv4Data = networkInterface->findProtocolData<Ipv4InterfaceData>())
-                return std::to_string(ipv4Data->getNetmask().getNetmaskLength());
+                return ipv4Data->getIPAddress().isUnspecified() ? "" : std::to_string(ipv4Data->getNetmask().getNetmaskLength());
 #endif // INET_WITH_IPv4
             return "";
         case '4':