Return "" when NetDev.IpCIDR is empty

Signed-off-by: Jonathon Anderson <janderson@ciq.com>

overlays/NetworkManager/internal/networkmanager_test.go

@@ -152,7 +152,6 @@ autoconnect=true
 [ethernet]
 [ipv4]
 method=manual
-address=<nil>
 route1=192.168.1.0/24,192.168.2.254
 [vlan]
 interface-name=eth0.902
@@ -176,7 +175,6 @@ autoconnect=true
 [ethernet]
 [ipv4]
 method=manual
-address=<nil>
 
 [ipv6]
 addr-gen-mode=stable-privacy

overlays/wicked/internal/wicked_test.go

@@ -153,7 +153,7 @@ This file is autogenerated by warewulf
   </ipv4>
   <ipv4:static>
     <address>
-      <local><nil></local>
+      <local></local>
     </address>
     <route>
       <destination>192.168.1.0/24</destination>
@@ -189,7 +189,7 @@ This file is autogenerated by warewulf
   </ipv4>
   <ipv4:static>
     <address>
-      <local><nil></local>
+      <local></local>
     </address>
   </ipv4:static>
   <ipv6>