PsqlDosBackend: database_name incorrectly marked optional (#6212)

The `database_name` key of the `Configuration` model was incorrectly
marked as accepting `None`, but it is actually required.

aiida/storage/psql_dos/backend.py

@@ -93,7 +93,7 @@ class Configuration(BaseModel):
         database_password: str = Field(
             title='PostgreSQL password', description='The password with which to connect to the PostgreSQL server.'
         )
-        database_name: Union[str, None] = Field(
+        database_name: str = Field(
             title='PostgreSQL database name', description='The name of the database in the PostgreSQL server.'
         )