Since it's returned by eglQueryStringiANGLE, and the doc says:
1. eGLQueryStringiANGLE returns a pointer to a static, null-terminated string
2. On failure, NULL is returned

So, we shouldn't be returning NULL when there is no failure.
This was breaking 'chrome --enable-gpu-service-logging'.

Bug: angleproject:3947
Change-Id: I2dcc7d479899dc1fc5fd59a502e84defe44f64c9
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2137939Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>