[daemon] Filter out empty MAC addresses.

Fixes issue 2 of #3195 (review)

src/daemon/daemon.cpp

@@ -1097,7 +1097,8 @@ std::unordered_set<std::string> mac_set_from(const mp::VMSpecs& spec)
     macs.insert(spec.default_mac_address);
 
     for (const auto& extra_iface : spec.extra_interfaces)
-        macs.insert(extra_iface.mac_address);
+        if (!extra_iface.mac_address.empty())
+            macs.insert(extra_iface.mac_address);
 
     return macs;
 }
@@ -1436,7 +1437,10 @@ mp::Daemon::Daemon(std::unique_ptr<const DaemonConfig> the_config)
         // only if this instance is not invalid.
         auto new_macs = mac_set_from(spec);
 
-        if (new_macs.size() <= spec.extra_interfaces.size() || !merge_if_disjoint(new_macs, allocated_mac_addrs))
+        if (new_macs.size() <= (size_t)count_if(spec.extra_interfaces.cbegin(),
+                                                spec.extra_interfaces.cend(),
+                                                [](const auto& i) { return !i.mac_address.empty(); }) ||
+            !merge_if_disjoint(new_macs, allocated_mac_addrs))
         {
             // There is at least one repeated address in new_macs.
             mpl::log(mpl::Level::warning, category, fmt::format("{} has repeated MAC addresses", name));