Fix OS.get_video_adapter_driver_info crash

doc/classes/OS.xml

@@ -451,7 +451,7 @@
 				Returns the video adapter driver name and version for the user's currently active graphics card.
 				The first element holds the driver name, such as [code]nvidia[/code], [code]amdgpu[/code], etc.
 				The second element holds the driver version. For e.g. the [code]nvidia[/code] driver on a Linux/BSD platform, the version is in the format [code]510.85.02[/code]. For Windows, the driver's format is [code]31.0.15.1659[/code].
-				[b]Note:[/b] This method is only supported on the platforms Linux/BSD and Windows. It returns an empty array on other platforms.
+				[b]Note:[/b] This method is only supported on the platforms Linux/BSD and Windows when not running in headless mode. It returns an empty array on other platforms.
 			</description>
 		</method>
 		<method name="has_environment" qualifiers="const">

platform/linuxbsd/os_linuxbsd.cpp

@@ -246,6 +246,10 @@ String OS_LinuxBSD::get_version() const {
 }
 
 Vector<String> OS_LinuxBSD::get_video_adapter_driver_info() const {
+	if (RenderingServer::get_singleton()->get_rendering_device() == nullptr) {
+		return Vector<String>();
+	}
+
 	const String rendering_device_name = RenderingServer::get_singleton()->get_rendering_device()->get_device_name(); // e.g. `NVIDIA GeForce GTX 970`
 	const String rendering_device_vendor = RenderingServer::get_singleton()->get_rendering_device()->get_device_vendor_name(); // e.g. `NVIDIA`
 	const String card_name = rendering_device_name.trim_prefix(rendering_device_vendor).strip_edges(); // -> `GeForce GTX 970`

platform/windows/os_windows.cpp

@@ -311,6 +311,10 @@ String OS_Windows::get_version() const {
 }
 
 Vector<String> OS_Windows::get_video_adapter_driver_info() const {
+	if (RenderingServer::get_singleton()->get_rendering_device() == nullptr) {
+		return Vector<String>();
+	}
+
 	REFCLSID clsid = CLSID_WbemLocator; // Unmarshaler CLSID
 	REFIID uuid = IID_IWbemLocator; // Interface UUID
 	IWbemLocator *wbemLocator = NULL; // to get the services