Don't throw when using normalize_name w/ ENSIP-15 if name is empty

- This causes methods that utilize normalize_name() internally to throw with a very obscure message that empty names aren't allowed. e.g. when calling ns.name(ens_address), if the reverse resolver isn't properly set up it will throw ``InvalidName`` with a message saying names can't be empty. This would create a lot of confusion.
- ``normalize_name_ensip15()`` method still raises ``InvalidName`` on empty name and this seems more appropriate when directly calling the method.

ens/utils.py

@@ -127,6 +127,9 @@ def normalize_name(name: str, ensip15: bool = False) -> str:
     :param bool ensip15: if True, normalize as per ENSIP-15
     :raises InvalidName: if ``name`` has invalid syntax
     """
+    if is_empty_name(name):
+        return ""
+
     if ensip15:
         return normalize_name_ensip15(name).as_text
 
@@ -137,9 +140,7 @@ def normalize_name(name: str, ensip15: bool = False) -> str:
         FutureWarning,
     )
 
-    if not name:
-        return name
-    elif isinstance(name, (bytes, bytearray)):
+    if isinstance(name, (bytes, bytearray)):
         name = name.decode("utf-8")
 
     try: