Roo::Base#each raises ArgumentError on empty sheets (last_row is nil)

[geremiasanti]

Hi, Same issue as #164 and #400, still happens on 3.0.0.

I've attached an example file, a valid xlsx, that causes the problem: empty_sheet.xlsx

reproduce with:

sheet = Roo::Spreadsheet.open("empty_sheet.xlsx", extension: ".xlsx").sheet(0)
sheet.first  #=> ArgumentError: comparison of Integer with nil
  failed

Happens because last_row is nil and 1.upto(nil) blows up in base.rb:286.

I think it should return zero instead (?)

[geremiasanti]

Created a pull request with a possible fix: #640