iputils: Return early when the tree is empty

This brings a small speed-up when the proxy protocol ACL is empty,
for example.

Signed-off-by: Remi Gacogne <remi.gacogne@powerdns.com>

Author: rgacogne

pdns/iputils.hh

@@ -1511,13 +1511,19 @@ public:
   //<! Returns "best match" for key_type, which might not be value
   [[nodiscard]] node_type* lookup(const key_type& value) const
   {
+    if (empty()) {
+      return nullptr;
+    }
     uint8_t max_bits = value.getBits();
     return lookupImpl(value, max_bits);
   }
 
   //<! Perform best match lookup for value, using at most max_bits
   [[nodiscard]] node_type* lookup(const ComboAddress& value, int max_bits = 128) const
   {
+    if (empty()) {
+      return nullptr;
+    }
     uint8_t addr_bits = value.getBits();
     if (max_bits < 0 || max_bits > addr_bits) {
       max_bits = addr_bits;