chroma: Do not proccess image if background image is not set
chroma: Do not proccess image if background image is not set

Change-Id: I66046895bc60301d9dfe01e62fca2cc40d98cf68

dfernandezlop/kms-chroma,shelsonjava/kms-chroma,Kurento/kms-chroma,Kurento/kms-chroma,dfernandezlop/kms-chroma,Kurento/kms-chroma,shelsonjava/kms-chroma,shelsonjava/kms-chroma,dfernandezlop/kms-chroma