I think the large $x$ limit is only zero if $a+b>3/2$: $$_1{F}_2[{1}; {a, b}; -x^2/4]=$$ $$=\pi^{-1/2}(-1)^{3/4} 2^{a+b-\frac{5}{2}} \Gamma (a) \Gamma (b) e^{-\frac{1}{2} i (\pi (a+b)+2 x)} \left(i e^{i \pi (a+b)}+e^{2 i x}\right) x^{-a-b+\frac{3}{2}}+{\cal O}(1/x^2)$$
For example, when $a=b=1/2$ the function increases as $\sqrt x$, see plot:
