Empty quotation marks usually indicate that something is being quoted but that the of the quotation is either unknown or not relevant.
In some cases, empty quotation marks may also be used for emphasis or to indicate that a word or phrase is being used in a nonliteral sense.
For example, the sentence "The socalled 'expert' didn't know what he was talking about" uses empty quotation marks to indicate that the word "expert" is being used ironically.