Unfortunately, having no file *is* a legal situation (and, from the processing logic, equivalent to an empty file)

Then change the logic, and make no file different to an empty file. That is, make it so that the create always creates a file, even if that file is empty. That way, your reader will always wait until there is a file before deciding upon its next action.

Having the absence of a file be logically equivalent to the presence of an empty file creates the situation where the reader takes the crash of the writer to mean the same thing as the writer creating an empty file. And that's just silly. It just creates a false dilemma.

With locking, the reader would have to wait for the writer to unlock the file. This will normally be a short period, but could potentially be forever if the creator crashed whilst holding the lock.

If the reader waits (with timeout) for the appearance of the file you have the same situation.