Check for blank cell type in addition to null check for mandatory cells

[virtual-machinist]

This change adds a check if the cell is blank, i.e. is not missing / null, but having CellType.BLANK.

This improves mandatory cell check behavior as described in #311 , and blank cells in XLS files and opened XLS/XLSX sheets also throw a PoijiMultiRowException.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.63%. Comparing base (0ee9a3f) to head (63d9c09).

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #314      +/-   ##
============================================
- Coverage     93.78%   93.63%   -0.16%     
+ Complexity      465      464       -1     
============================================
  Files            44       44              
  Lines          1304     1304              
  Branches        161      161              
============================================
- Hits           1223     1221       -2     
- Misses           45       47       +2     
  Partials         36       36              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[virtual-machinist]

Closing this, since will probably opt out for the better Row.MissingCellPolicy.RETURN_BLANK_AS_NULL option one way or another.