Click just after time zero in each of the tracks that does not have audio starting at zero, then hold SHIFT and press HOME to select from the click point to time zero, then Generate > Silence. Or just click and drag to zero then generate silence.
If multiple adjacent tracks don’t start at zero, then after SHIFT + HOME, you can hold SHIFT and press DOWN arrow to extend the selection into the other tracks, then Generate > Silence.
After that you can select a track to export then use File > Export Selection… or if you want to export all the tracks at one go you can use File > Export Multiple… .
Would you like there to be a preference to export silence at the start of tracks whose audio starts after time zero?
Gale