Make movit_mix ignore empty "mix" parameter strings.
Make movit_mix ignore empty "mix" parameter strings.

Previously, the parameter would have to be entirely unset
(ie., NULL) for the transition progress to be used, which
was suboptimal in some cases.

xzhavilla/mlt,j-b-m/mlt,wideioltd/mlt,mltframework/mlt,anba8005/mlt,mltframework/mlt,anba8005/mlt,mltframework/mlt,j-b-m/mlt,siddharudh/mlt,siddharudh/mlt,zzhhui/mlt,anba8005/mlt,xzhavilla/mlt,siddharudh/mlt,wideioltd/mlt,zzhhui/mlt,mltframework/mlt,anba8005/mlt,siddharudh/mlt,zzhhui/mlt,zzhhui/mlt,mltframework/mlt,siddharudh/mlt,xzhavilla/mlt,xzhavilla/mlt,anba8005/mlt,wideioltd/mlt,xzhavilla/mlt,j-b-m/mlt,xzhavilla/mlt,xzhavilla/mlt,wideioltd/mlt,j-b-m/mlt,wideioltd/mlt,xzhavilla/mlt,wideioltd/mlt,j-b-m/mlt,mltframework/mlt,siddharudh/mlt,anba8005/mlt,zzhhui/mlt,zzhhui/mlt,mltframework/mlt,siddharudh/mlt,xzhavilla/mlt,j-b-m/mlt,anba8005/mlt,wideioltd/mlt,j-b-m/mlt,siddharudh/mlt,j-b-m/mlt,siddharudh/mlt,anba8005/mlt,mltframework/mlt,zzhhui/mlt,wideioltd/mlt,j-b-m/mlt,j-b-m/mlt,mltframework/mlt,anba8005/mlt,zzhhui/mlt,mltframework/mlt,zzhhui/mlt,wideioltd/mlt