Activate 'no_implicit_optional' mypy check

Author: schnellerhase

pyproject.toml

@@ -67,7 +67,7 @@ log_cli = true
 # Goal would be to make all of the below True long-term
 disallow_untyped_defs = false
 disallow_any_unimported = false
-no_implicit_optional = false
+no_implicit_optional = true
 check_untyped_defs = false
 warn_return_any = true
 warn_unused_ignores = true