haruspex said:
Since only extrema are of interest, it might not be necessary to solve the integral.
Since the OP only asked if it is zero or not at one point along the radius, you would not have to actually do any integrals but just set the problem up. I found the formula worked out here*. The answer does involve Elliptical integrals of the first and second kind. If it wasn't enough to just look at the formula and see it is not zero, one can evaluate the elliptical integrals by lookup table at that point and see the value is not zero. I still think none of this is necessary based on the previous strategy.
$$E_r = \frac{2\pi R \lambda}{ 4πε_0} \frac{2}{ ^{\large \pi q^\frac{3 }{2}} (1−µ) }\frac{1 }{µ} (2R(1−µ) K(k)−(2R−µ(r +R))E(k))$$
where ##~~q = r^2 + R^2 +2rR ~~; k = \sqrt\mu## and ##\mu = \large \frac{4rR}{q}## and ##~~K(k) , E(k)## are the complete elliptical integrals of the first and second kind.
This can be simplified by using ##a = \large\frac{r}{R}## and noticing ##q = (R + r)^2 = R^2(1 + a)^2##
we get finally;
$$E_r = \frac{2 \lambda}{ 4πε_0 R} \left[\frac{1}{a} \left(\frac{K(k)}{(1+a)} - \frac{E(k)}{(1-a)}\right)\right],~~~a>0$$
or in the form with ##k_e =\large \frac{1}{4πε_0}## and ##Q = 2\pi R \lambda##;
$$E_r = \frac{ k_e Q}{ \pi R^2} \left[\frac{1}{a} \left(\frac{K(k)}{(1+a)} - \frac{E(k)}{(1-a)}\right)\right],~~~a>0$$
For ##a = \large \frac{2}{3}##, the ratio## \frac{ \Large E_r R^2}{\Large k_e Q}##gives ~-0.693 which is not zero.
since ##a =\Large \frac{r}{R}## and ##k = \Large \frac{2\sqrt a}{1+a}## as ##a → 0, E_r → 0## as ##a →1## ##E_r## diverges.
then as ##a, r →∞, k → 0, K(k), E(k) → \frac{\Large\pi}{2}## and ##E_r → \frac{ \large k_e Q}{ \ \Large r^2} ##
Here is a plot of what the field looks like. The black curve is the equivalent plot of a point charge equal to the ring for reference. The ##x## axis is distance of the point ##P## from the center of the ring in units of ##R##.
*
http://www.mare.ee/indrek/ephi/efield_ring_of_charge.pdf