ImageAlgo : Add missing early returns for when data window is empty.
ImageAlgo : Add missing early returns for when data window is empty.

These were requested during review but it seems like only one got added.

appleseedhq/gaffer,johnhaddon/gaffer,appleseedhq/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,hradec/gaffer,ivanimanishi/gaffer,ImageEngine/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,ivanimanishi/gaffer,ImageEngine/gaffer,appleseedhq/gaffer,boberfly/gaffer,lucienfostier/gaffer,chippey/gaffer,lucienfostier/gaffer,GafferHQ/gaffer,chippey/gaffer,appleseedhq/gaffer,ivanimanishi/gaffer,ImageEngine/gaffer,lucienfostier/gaffer,chippey/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,chippey/gaffer,ImageEngine/gaffer,boberfly/gaffer,hradec/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,hradec/gaffer,hradec/gaffer,boberfly/gaffer,GafferHQ/gaffer,boberfly/gaffer,andrewkaufman/gaffer,ivanimanishi/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,hradec/gaffer,GafferHQ/gaffer