Fix EMA config behaviour

[jilek-josef]

EMA could not be explicitly set to false, always defaulted to True, so long as ema_config was not None

new behaviour if ema config exists and use_ema is not false result to True, otherwise False (behaviour before fix -> if ema_config is not None use_ema is always True, even when set false, otherwise False)

[inflamously]

What I've also noticed is that when storing ema within a job, sometimes the config acquired two values like: