Assignment from empty pool gives weird error message
```
kll@lingloi320 ~ $ nipap address add family ipv6 from-pool FOO description "BAR"
Could not add prefix to NIPAP: VRF must be the same as the pools implied VRF
```

After adding a prefix to the pool it all works well. This is not the error message I would expect from an empty pool!

diff --git a/nipap/nipap/backend.py b/nipap/nipap/backend.py
index f2eb90d2..b3d761fe 100644
--- a/nipap/nipap/backend.py
+++ b/nipap/nipap/backend.py
@@ -2291,6 +2291,12 @@ class Nipap:
 
             if 'from-pool' in args:
                 from_pool = self._get_pool(auth, args['from-pool'])
+
+                # make sure there are prefixes in pool, if any prefix is present
+                # than the implied-vrf is set, otherwise the pool is empty
+                if from_pool['vrf_id'] is None:
+                    raise NipapInputError('No prefixes in pool')
+
                 # set default type from pool if missing
                 if 'type' not in attr:
                     attr['type'] = from_pool['default_type']
['nipap/nipap/backend.py']