CI: Fail build on "UserWarning: Code input with no code at..."
Blank lines in ipython blocks cause the warning "UserWarning: Code input with no code at..." to be emitted when building the docs. #41400 removed the blank lines so there are currently no warnings, but they will likely reappear, specially since double blank lines around classes/functions looks correct. Should we fail the doc build on such a warning (not sure why that doesn't already happen) or enforce this through style checks?