Add "None" condition tests for duration_string and duration_decimal.
Add "None" condition tests for duration_string and duration_decimal.

cdubz/timestrap,Leahelisabeth/timestrap,overshard/timestrap,Leahelisabeth/timestrap,overshard/timestrap,overshard/timestrap,muhleder/timestrap,Leahelisabeth/timestrap,Leahelisabeth/timestrap,cdubz/timestrap,cdubz/timestrap,muhleder/timestrap,muhleder/timestrap