Issue 1338

Modified the empty case check in alpha_paginate() in app/models/grade_entry_form.rb
The old code used the condition "total_pages == 0" to check if there were no students to display on the spreadsheet. However, the method was given total_pages = 1 in that scenario.

We added another condition to check if the "all_grade_entry_students" array was empty instead. 

To test this issue, we created a new grader user with the MarkUs GUI and switched to that user before displaying the spreadsheets.