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)
(cherry picked from commit fb8b0b6)

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;