Fix docs Makefile for case where a notebook doesn't have pictures
Fix docs Makefile for case where a notebook doesn't have pictures

Tim's new notebook is pure text apparently.

SKA-ScienceDataProcessor/algorithm-reference-library,SKA-ScienceDataProcessor/algorithm-reference-library,SKA-ScienceDataProcessor/algorithm-reference-library,SKA-ScienceDataProcessor/algorithm-reference-library,SKA-ScienceDataProcessor/algorithm-reference-library