Fix types and add test for concat (#125)

Arguments for `concat` are allowed to be null, so this is just a simple fix for
 the existing stub.

Author: ckarnell

sqlalchemy-stubs/sql/functions.pyi

@@ -112,7 +112,7 @@ class sum(ReturnTypeFromArgs[Any]): ...
 
 class now(GenericFunction[datetime]): ...
 
-class concat(GenericFunction[Text]): ...
+class concat(GenericFunction[Optional[Text]]): ...
 
 class char_length(GenericFunction[int]): ...
 

test/test-data/sqlalchemy-functions.test

@@ -16,3 +16,22 @@ users_table = Table(
 
 query = users_table.update().values(user=functions.concat(users_table.c.user, "@domain.com"))
 [out]
+
+[case testConcatFunctionNullableString]
+from sqlalchemy.sql import functions
+
+from sqlalchemy import Column, Table
+from sqlalchemy.types import Integer, String
+from sqlalchemy import MetaData
+
+metadata = MetaData()
+
+users_table = Table(
+    "users",
+    metadata,
+    Column("id", Integer, primary_key=True, autoincrement=True),
+    Column("user", String(42), unique=True, nullable=True),
+)
+
+query = users_table.update().values(user=functions.concat(users_table.c.user, "@domain.com"))
+[out]