Vulkan: Fix nullptr in Diagnostics - VK Spec for VkDebugUtilsMessengerCallbackDataEXT says pMessageIdName may be nullptr and pMessage must not be a nullptr - This was enforced by the following change Vulkan: Don't crash on diagnostics with null pMessageIdName - But this was susequently broken by the following change Vulkan: Enable sync validation - This change adds a check to skip accessing callbackData->pMessageIdName if it is null Bug: angleproject:5290 Change-Id: I82dd225d875e233530158013efc280e979f82a5b Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5238650 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Brandon Schade <b.schade@samsung.com>