Previously, an empty row would mess with the parser: if we're in an
empty row, our helper methods don't detect us as being in the "row" _or_
in the "sheetData". So `has_cell()` would return false when it
shouldn't. Similarly, `read_cell()` wouldn't skip rows; so `read_cell()`
would return an invalid cell when placed in an empty row, causing a
segfault when the caller tried to use the cell.
Callers must take care to call `has_next()` before `read_next()`. In
the future, perhaps we can make `read_next()` return a `std::optional`
and nix `has_next()` altogether?
[Closes#492]
