Fix EnvironmentRecord caching for core segments without package handle

Core records, that leave `$pkgHandle` at `Environment::getRecord($segment, $pkgHandle = false)` undefined, do not get fetched from cache (`Environment::$cachedOverrides`), since `$pkgHandle` defaults to `false`, core packages are cached with empty string as a key, and hence the `isset` check against `$cachedOverrides` did not match. As a result `EnvironmentRecord` returns a new object on every call to `getRecord`.