Generic File Parser: Improve empty column handling

[dwaring87]

Description

This updates the Generic File Parser and the Excel and Plain text plugins to improve handling of empty columns.

If a column does not have a header, it will not process that column when processing each of the rows.
It will stop checking columns after 5 blank columns.

Related to #5294

Checklist

• Refactoring only
• Documentation only
• Fixture update only
• Bug fix
  • The relevant issue has been closed.
  • Further work is required.
• New feature
  • Relevant tests have been created and run.
  • Data was added to the fixture
    • Data was added via a patch in /t/data/fixture/patches/.
  • User-Facing Change
    • The user manual in /docs has been updated.
  • Any new Perl has been documented using perldoc.
  • Any new JavaScript has been documented using JSDoc.
  • Any new legacy JavaScript has been moved from /js to /js/source/legacy.

[chris263]

:-)