[FIX] stock: index error in stock_move while split_lines is empty

An IndexError `list index out of range`  trace back that occurs in stock/
stock_move : _onchange_move_line_ids was caught by sentry.
because split_lines list is empty when user enter blank entry  in value.

So hear we check the split_lines is empty or not.

sentry - 3942491608

closes odoo/odoo#113353

Signed-off-by: Adrien Widart <awt@odoo.com>