SystemError.build_message: Also restrict message to String?.

src/system_error.cr

@@ -85,7 +85,7 @@ module SystemError
     #
     # By default it returns the original message unchanged. But that could be
     # customized based on the keyword arguments passed to `from_errno` or `from_winerror`.
-    protected def build_message(message, **opts) : String?
+    protected def build_message(message : String?, **opts) : String?
       message
     end