network-generator: drop wrong warning for rd.peerdns without value

(cherry picked from commit 2a774f0)
(cherry picked from commit 56cc8ac)
(cherry picked from commit 2e956d0)
(cherry picked from commit d3132ec)

Author: yuwata

src/network/generator/network-generator.c

@@ -919,10 +919,7 @@ static int parse_cmdline_rd_peerdns(Context *context, const char *key, const cha
         assert(context);
         assert(key);
 
-        if (proc_cmdline_value_missing(key, value))
-                return network_set_dhcp_use_dns(context, "", true);
-
-        r = parse_boolean(value);
+        r = value ? parse_boolean(value) : true;
         if (r < 0)
                 return r;