Count cache configuration does not work as expected

In the constructor of SourcePluginBase the cache_key configuration value for the count cache is used in a surprising way:

$this->cacheKey = !empty($configuration['cache_key']) ? !empty($configuration['cache_key']) : NULL;

As a result when I set a cache key in my migration configurations they all share the same count cache. I assume that it is a typo and instead it should be:

$this->cacheKey = !empty($configuration['cache_key']) ? $configuration['cache_key'] : NULL;

I can provide a patch if that is the correct behaviour.

I have also noticed that the default cache key provided is the same for all migrations sharing a source plugin.

if (!isset($this->cacheKey)) {
  $this->cacheKey = hash('sha256', $this->getPluginId());
}

I am a not sure how to obtain a safe default.