Graduated symbology makes some polygons disappear

Question

I am working with QGIS 3.8 and I have layer of the world countries which I did Join in order to create data about each country (e.g population, corona cases and more).

Whenever I try to create graduated symbology based on one of the columns, some of the countries polygons dissapear, seems like only the ones that has no data. So as you can see, when I have sungle symbol the map looks like this:

But when I try to create graduted symbology based on column where not all the coountries has data I get this:

I believe the countries that dissapere are the ones that have no values, meaning, if it has 0 it shows the country but if in the table it's empty it has nothing.

Is there any easy way to fix it, which doesen't require me to go to Excel and write zero in any country that has no value?

Is there any way to make those values null and also add them with grey color to the map?

Answer 1

You first begin with a rule-based symbology with two rules :

one excluding your null cases and the second including all the other cases.

You then choose "refine current rule" (on the second rule) and add your categories...