Fix#2810. Repairing some Phpstan diagnostics, used `?:` rather than `??` in a few places.
2 different Html modules are affected. Also, Ods Reader, but its problem is with sheet title rather than cell contents. And, as it turns out, Ods Reader was already not handling sheets with a title of `0` correctly - it made a truthy test before setting sheet title. That is now changed to truthy or numeric. Other readers are not susceptible to this problem. Tests are added.
oleibman
Adrien Crivelli