Allow empty values so that upstream= can disable the default

llarp/config/config.cpp

@@ -366,7 +366,8 @@ namespace llarp
 
     conf.defineOption<std::string>(
         "dns", "upstream", false, true, DefaultUpstreamDNS, [=](std::string arg) {
-          m_upstreamDNS.push_back(parseDNSAddr(std::move(arg)));
+          if (!arg.empty())
+            m_upstreamDNS.push_back(parseDNSAddr(std::move(arg)));
         });
 
     conf.defineOption<std::string>("dns", "bind", false, "127.3.2.1:53", [=](std::string arg) {

llarp/config/ini.cpp

@@ -120,7 +120,7 @@ namespace llarp
             x->remove_suffix(1);
         }
 
-        if (k.size() == 0 || v.size() == 0)
+        if (k.size() == 0)
         {
           LogError(m_FileName, " invalid line (", lineno, "): '", line, "'");
           return false;