Here is a patch to add an empty item in category menu list:

From 9c2e6b6ec1b7a9b7d703ead1ff3b1ef1c106f9e7 Mon Sep 17 00:00:00 2001
Message-Id: <9c2e6b6ec1b7a9b7d703ead1ff3b1ef1c106f9e7.1576761586.git.martin.leusch@imrcom.fr>
From: Martin LEUSCH <martin.leusch@imrcom.fr>
Date: Thu, 19 Dec 2019 14:19:36 +0100
Subject: [PATCH] Add an empty item in category menu list

.../content/sogo-connector/addressbook/common-card-overlay.js | 3 +++
1 file changed, 3 insertions(+)

diff --git a/chrome/content/sogo-connector/addressbook/common-card-overlay.js b/chrome/content/sogo-connector/addressbook/common-card-overlay.js
index cc72457..6f1bc38 100644
--- a/chrome/content/sogo-connector/addressbook/common-card-overlay.js
+++ b/chrome/content/sogo-connector/addressbook/common-card-overlay.js
@@ -114,6 +114,9 @@ function SCResetCategoriesMenu(menu, catValue) {
//}
menu.removeAllItems();

• // Add an empty item to menu for no category
• let itemNone = menu.appendItem("");

• //let menuPopup = document.createElement("menupopup");
  for (let catName of gSCCardValues.categories) {
  //let item = document.createElement("menuitem");
  --
  2.24.0

ps: I had an error by trying to upload the patch file as attachment.