How does X chromosome monosomy occur
Most often, it seems that Turner syndrome is due to fertilization by a sperm cell lacking a sex chromosome. I wouldn't describe such a cell as "empty": it still has the other 22 chromosomes, but in meiosis the sex chromosomes failed to segregate appropriately.

Monroy et al 2002 note that of 10 people with complete monosomy X, 9 had their single chromosome from the mother. 

Monroy, N., López, M., Cervantes, A., García‐Cruz, D., Zafra, G., Canún, S., ... &amp; Kofman‐Alfaro, S. (2002). Microsatellite analysis in Turner syndrome: parental origin of X chromosomes and possible mechanism of formation of abnormal chromosomes. American journal of medical genetics, 107(3), 181-189.