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

KurentoLegacy/gst-kurento-plugins,ESTOS/kms-core,KurentoLegacy/gst-kurento-plugins,shelsonjava/kms-core,Kurento/kms-core,Kurento/kms-core,shelsonjava/kms-core,ESTOS/kms-core,Kurento/kms-core,shelsonjava/kms-core,KurentoLegacy/gst-kurento-plugins,ESTOS/kms-core,TribeMedia/kms-core,TribeMedia/kms-core,KurentoLegacy/gst-kurento-plugins,TribeMedia/kms-core,Kurento/kms-core,shelsonjava/kms-core,TribeMedia/kms-core,ESTOS/kms-core