schemas: codexentry: NearestDestination *can* be an empty string.

We had one user with EDMC trying to send:

```python
OrderedDict([('$schemaRef',
'https://eddn.edcd.io/schemas/codexentry/1'), ('message',
OrderedDict([('timestamp', '2021-10-26T11:37:57Z'), ('event',
'CodexEntry'), ('EntryID', 2420702), ('Name',
'$Codex_Ent_Stratum_07_K_Name;'), ('SubCategory',
'$Codex_SubCategory_Organic_Structures;'), ('Category',
'$Codex_Category_Biology;'), ('Region', '$Codex_RegionName_18;'),
('System', 'Xi Ophiuchi'), ('SystemAddress', 2072061675891),
('NearestDestination', ''), ('Latitude', 79.352364), ('Longitude',
121.434044), ('horizons', True), ('odyssey', True), ('StarPos', [-3.875,
8.15625, 55.875]), ('BodyName', 'Xi Ophiuchi B 5'), ('BodyID', 14)]))])
```
as the message.  The EDDN Gateway then complains:

`FAIL: [<ValidationError: "'' is too short">]`

schemas/codexentry-v1.0.json

@@ -92,8 +92,7 @@
                     "minLength"     : 1
                 },
                 "NearestDestination": {
-                    "type"          : "string",
-                    "minLength"     : 1
+                    "type"          : "string"
                 },
                 "VoucherAmount": {
                     "type"          : "integer"