Fixed Functions::ifCondition for allowing empty condition ("") and different than empty condition ("<>")

[jorgecasas]

This is:

- [X] a bugfix
- [ ] a new feature

Checklist:

• Changes are covered by unit tests
• 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 if necessary
• [] Documentation is updated as necessary

Why this change is needed?

In cells with formulas containing conditions like =IFSUM(A1:A3;"";B1:B3) to sum cells from range A1:A3 with empty value in range B1:B3, the function Functions::ifCondition() create in this case the code ="""" instead of ="", so it didn't work.

Added test to check condition empty ("") and also condition different value than empty ("<>")

[PowerKiKi]

Thanks for the PR !