Fix attendance sheet filtering (#41)

This bug was introduced in aa0d578 - it
used to continue out of an outer loop to the next sheet, but now the
continue just skips to the next row.

This function now handles just one sheet, so if a sheet is not a
register sheet, just return immediately.

Author: illicitonion

src/register.rs

@@ -105,7 +105,7 @@ fn read_module(
     for (row_number, cells) in sheet_data.into_iter().enumerate() {
         // Some sheets have documentation or pivot table
         if row_number == 0 && !cells.is_empty() && cell_string(&cells[0]) != "Name" {
-            continue;
+            return Ok(sprints);
         }
         if cells.len() < 7 {
             return Err(anyhow::anyhow!(