Xls Writer Parser Mis-handling TRUE/FALSE As VLOOKUP Arguments

[oleibman]

Fix #4331. Parser treats TRUE/FALSE as functions rather than constants, which is nominally harmless, but it then expects an argument count of 0 and instead sees null-string. Changed to recognize this situation and leave TRUE/FALSE/TRUE()/FALSE() unchanged.

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary