Yes, it is legal. A summons is not invalidated for a harmless error, and this error would be regarded as harmless. It is also not invalidated for an easily cured or corrected deficiency, and this could be fixed by simply letting the officer "amend" the citation to include that signature. Indeed, even if this citation were struck down for a procedural irregularity or deficiency, it would be dismissed "without prejudice," which means they could simply issue you a new summons for the same infraction.