Answer
$B(r) = 0$
Work Step by Step
We can find an expression for the current enclosed in a loop of radius $r$ where $r \gt a$:
$i_{enc} = i-i = 0$
We can find an expression for $B(r)$:
$\int~B\cdot ds = \mu_0~i_{enc}$
$B\cdot 2\pi~r = (\mu_0)~(0)$
$B(r) = \frac{0}{2\pi~r}$
$B(r) = 0$
