In the next version (5.8), which is planned for release by the end of this week, the "Rule" action will no longer evaluate "THEN" expressions if the condition is not true. This will cover most cases where the columnexists() function is used.
In the image below, the "Rule" action works correctly in v5.8 even if the [revenue] column is missing.
Improving the IF/THEN operator and the if() function would require deeper changes and will be done later. For now, the "Rule" action should work in most cases.