Curves of given $p$-rank with trivial automorphism group

Let $k$ be an algebraically closed field of characteristic $p >0$. Suppose $g \geq 3$ and $0 \leq f \leq g$. We prove there is a smooth projective $k$-curve of genus $g$ and $p$-rank $f$ with no non-trivial automorphisms. In addition, we prove there is a smooth projective hyperelliptic $k$-curve of genus $g$ and $p$-rank $f$ whose only non-trivial automorphism is the hyperelliptic involution. The proof involves computations about the dimension of the moduli space of (hyperelliptic) $k$-curves of genus $g$ and $p$-rank $f$ with extra automorphisms.