transition_composite.c: allow removing of luma file by passing an empty name

[melted] / profiles / quarter_pal_wide

description=Quarter Square PAL Widescreen
frame_rate_num=25
frame_rate_den=1
width=512
height=288
progressive=1
sample_aspect_num=1
sample_aspect_den=1
display_aspect_num=16
display_aspect_den=9