Windows: Validate that the font family name exists before trying to get the font family

I didn't have the first emoji font installed, which led an an exception when calling font_collection->GetFontFamily() no the new rows.

platform/windows/os_windows.cpp

@@ -1059,7 +1059,7 @@ String OS_Windows::get_system_font_path(const String &p_font_name, int p_weight,
 	UINT32 index = 0;
 	BOOL exists = false;
 	HRESULT hr = font_collection->FindFamilyName((const WCHAR *)font_name.utf16().get_data(), &index, &exists);
-	if (FAILED(hr)) {
+	if (FAILED(hr) || !exists) {
 		return String();
 	}