T5889: Fix migration scripts nat 5-to-6

The current migration drop interface name for NAT where not should
```
nat {
    source {
        rule 100 {
            outbound-interface {
                name "eth0"
        ...
        }
    }
}
```
After migration we lost interface:
/home/vyos# /opt/vyatta/etc/config-migrate/migrate/nat/5-to-6 tmp.conf
/home/vyos#
/home/vyos# cat tmp.conf | grep "nat {" -A 10
nat {
    source {
        rule 100 {
            outbound-interface {
                interface-name ""
         ...
        }
    }
}
```
This commit fixes it.

1 files changed, 3 insertions, 2 deletions

diff --git a/src/migration-scripts/nat/5-to-6 b/src/migration-scripts/nat/5-to-6
index de3830582..c83b93d84 100755
--- a/src/migration-scripts/nat/5-to-6
+++ b/src/migration-scripts/nat/5-to-6
@@ -51,8 +51,9 @@ for direction in ['source', 'destination']:
         for iface in ['inbound-interface','outbound-interface']:
             if config.exists(base + [iface]):
                 tmp = config.return_value(base + [iface])
-                config.delete(base + [iface])
-                config.set(base + [iface, 'interface-name'], value=tmp)
+                if tmp:
+                    config.delete(base + [iface])
+                    config.set(base + [iface, 'interface-name'], value=tmp)
 
 try:
     with open(file_name, 'w') as f: