How can I avoid that a small bit of filament sticks out of the nozzle during heating? So when my Prusa I3 is heating up, a small bit of filament is coming out of the nozzle, forming a small string at.

When the printing starts, this results in a small plastic ball on the bed, impacting the overall printing process.

How can I avoid this?
I normally print a skirt. This acts as a quality check for: flow rate; bed adhesion; bed level; and proper zero position in the Z.