Fix ArrayOfEnum's handling of empty arrays.

Prior to this change a value of `'{}'` would split into the list `['']`.
(cherry picked from commit a2badb751f)

lib/sqlalchemy/dialects/postgresql/base.py

@@ -544,7 +544,7 @@ use the following workaround type::
 
             def handle_raw_string(value):
                 inner = re.match(r"^{(.*)}$", value).group(1)
-                return inner.split(",")
+                return inner.split(",") if inner else []
 
             def process(value):
                 if value is None: