To hide NULL on the chart, change format of the filed SUM(Pears) to hide marks of special values (e.g. NULL).
I believe you have tried this, but this will remove empty periods and the axis don't match with the upper chart.
To avoid this, I've put 'apple' on the same chart and used dual axis, synchronize, then change the color to opacity 0%.
I hope this is what you need.
example v2 2018.1.twbx 28.3 KB
Good workaround thanks. It does work. The line which has transparency can still be selected of course which is not totally perfect, but it does the job for what i need. Thank you very much!!