Improve 'format:hostname' to handle "" and "."

The `fqdn` library used to provide `"hostname"` format validation
(when installed) does not properly handle some known inputs,
including `""` and `"."`.

On these strings, the library improperly emits `ValueError`.

Given that the library no longer appears to be receiving updates,
there are two potential workarounds in `jsonschema`:

1. add `ValueError` handling via `raises=...`
2. add explicit handling for known patterns before calling out to
   `fqdn`

This changeset implements (1) only, adding handling for `ValueError`.
New test cases are added for `""` and `"."`.

resolves #1121

Author: sirosen

json/tests/draft7/optional/format/hostname.json

@@ -112,6 +112,16 @@
                 "description": "single label ending with digit",
                 "data": "hostnam3",
                 "valid": true
+            },
+            {
+                "description": "empty string",
+                "data": "",
+                "valid": false
+            },
+            {
+                "description": "single dot",
+                "data": ".",
+                "valid": false
             }
         ]
     }

jsonschema/_format.py

@@ -272,6 +272,10 @@ def is_ipv6(instance: object) -> bool:
         draft7="hostname",
         draft201909="hostname",
         draft202012="hostname",
+        # fqdn.FQDN("") raises a ValueError due to a bug
+        # however, it's not clear when or if that will be fixed, so catch it
+        # here for now
+        raises=ValueError,
     )
     def is_host_name(instance: object) -> bool:
         if not isinstance(instance, str):