Default direction of stems on the center line of the staff

The default direction of stems on the center line of the staff is set by the Stem property neutral-direction.

\version "2.24"

\relative c'' {
  a4 b c b
  \override Stem.neutral-direction = #up
  a4 b c b
  \override Stem.neutral-direction = #down
  a4 b c b
}