I don't think there are areas (on the Francés) that have a particularly high risk for being struck by lightning.
I am very afraid of thunderstorms. So I totally understand the question.
But the good thing about the Camino is, that it leads through many towns and villages. Civilisation is usually near. So if you see a storm coming up, it is easy to stop walking and either wait in a café until it is over, check into an albergue and stop walking for the day, or walk into a bar and call for a taxi to bring you to your destination of the day.
Watch the weather radar and lightning maps, they're usually very accurate.
In Spain I got into a storm twice, both times because I ignored the forming storm, walking with others who didn't see the risk and said "don't worry it is still far away"... both times in Galicia in summer (warm temperatures + hills + lots of rain = good for lightning).
There's one section in the meseta (17km without shelter, all flat, you're the highest point) where I would check before I walk if a storm might be approaching. Same in the pyrenees. Mountains are always good for storms in the afternoon. Look at the weather report and / or ask locals, or finish walking early in the day.
But overall, don't worry too much.
Because of the good infrastructure it is very easy to plan around any storms, and they're quite rare in my experience, anyway, at least on the Francés. And I say that as someone who is horribly afraid of lightning and will take a day off walking when I know there's a risk of it in the forecast.
Buen Camino!