Feature::condition must not be NULL

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/+/2137939
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>

include/platform/Feature.h

@@ -139,7 +139,7 @@ inline Feature::Feature(const char *name,
       description(description),
       bug(bug),
       enabled(false),
-      condition(nullptr)
+      condition("")
 {
     if (mapPtr != nullptr)
     {