In a If activity, when using the Expression:
Length('#testing_var#')=0
And #testing_var# does not have a value assigned, meaning, the "Variable Value" input was left BLANKED / EMPTY when creating the variable:
1. - For v3.8 it says: Condition "Length('#testing_var#')=0" evaluated to True
2. - For v4.2 it says: Expression can't be empty
And the automation fails.