nixos/networking/keepalived: do not emit `unicastPeers` when there are none

In the past, we emitted `unicast_peer` block even with empty unicast peers list.

This now results in:
```
Keepalived_vrrp[392762]: A unicast keyword has been specified without any unicast peers. Defaulting to multicast. This usage is deprecated - please update your configuration.
```

We do not emit it anymore to avoid this warning.

1 files changed, 5 insertions, 3 deletions

diff --git a/nixos/modules/services/networking/keepalived/default.nix b/nixos/modules/services/networking/keepalived/default.nix
index 429a47c3962c..bf5e96f001a8 100644
--- a/nixos/modules/services/networking/keepalived/default.nix
+++ b/nixos/modules/services/networking/keepalived/default.nix
@@ -59,9 +59,11 @@ let
         ${optionalString i.vmacXmitBase "vmac_xmit_base"}
 
         ${optionalString (i.unicastSrcIp != null) "unicast_src_ip ${i.unicastSrcIp}"}
-        unicast_peer {
-          ${concatStringsSep "\n" i.unicastPeers}
-        }
+        ${optionalString (builtins.length i.unicastPeers > 0) ''
+          unicast_peer {
+            ${concatStringsSep "\n" i.unicastPeers}
+          }
+        ''}
 
         virtual_ipaddress {
           ${concatMapStringsSep "\n" virtualIpLine i.virtualIps}