fix(fields): allow for empty dates subtraction [CLK-578056]

[mysza]

Summary

The formula parser clients use null, undefined, '' and false values to indicate "unset" field value (the last one is used by date type).

This change makes the parser less strict about the arguments:

• in addition (+), if value is not set, we treat it as 0
• other operators (subtraction, division, multiplication, power): if any of the operators is not set, we return Number.NaN, which is presented to the user as -, i.e. value not set, as we can't calculate it

Tests

• updated tests
• CI/CD ✅