Complexity calculation of a recursive function with additional O(n) complexity
The question is absolutely not answerable without knowing what aFunctionWhichComplexityisO(n) returns. 

Assuming that the function takes x as an argument, it might for example check if x contains the decimal representation of an integer y, and returns a string with the decimal representation of y-1 if y > 0, and null if y ≤ 0 or x does not contain the decimal representation of an integer. 

In that case if x contains n characters, the time is $O (n·10^n)$. 

Or the function always returns null, then the time is $O (n)$.