"Not an empty slogan" means "not an empty slogan". Double negation means to deny twice in a sentence. Obviously, this sentence has only one negative word "no", that is, it is only denied once.

Double negation literally means that there are two negative words in a sentence, which strengthens the positive meaning. Double negation, compared with simple negative sentences, expresses positive meaning in the form of negation plus negation. General grammar books generally think that the function of double negation is to strengthen tone.

Examples;

No one is not moved by the story of "Old Seagull".

I have to admit it belongs to me.

There is no child in kindergarten who doesn't like Mr. Wang.