[Bug] Empty values in PATCH `/user/<id>` are not propagated to database
**Describe the bug**
If we do a PATCH `/user/<id>` and update a field with empty value, that field is not updated in the database.
E.g. if a user has firstname 'Alice' and we try to edit that to '' (empty string), the firstname will not be updated.
Similar for other fields in the user endpoint.
We should also check whether this may be the case for other PATCH endpoints.

**Expected behavior**
If someone wants to put an empty string in a PATCH request, that should be done (given that it's allowed for that field)

**Details**
 - v6 version 3.3.7

diff --git a/vantage6-server/vantage6/server/resource/user.py b/vantage6-server/vantage6/server/resource/user.py
index b656a509..d945623e 100644
--- a/vantage6-server/vantage6/server/resource/user.py
+++ b/vantage6-server/vantage6/server/resource/user.py
@@ -556,19 +556,27 @@ class User(UserBase):
             return {"msg": "You cannot change your password here!"}, \\
                 HTTPStatus.BAD_REQUEST
 
-        if data["username"]:
-            if (user.username != data["username"] and
+        if data["username"] is not None:
+            if data["username"] == '':
+                return {
+                    "msg": "Empty username is not allowed!"
+                }, HTTPStatus.BAD_REQUEST
+            elif (user.username != data["username"] and
                     db.User.exists("username", data["username"])):
                 return {
                     "msg": "User with that username already exists"
                 }, HTTPStatus.BAD_REQUEST
             user.username = data["username"]
-        if data["firstname"]:
+        if data["firstname"] is not None:
             user.firstname = data["firstname"]
-        if data["lastname"]:
+        if data["lastname"] is not None:
             user.lastname = data["lastname"]
-        if data["email"]:
-            if (user.email != data["email"] and
+        if data["email"] is not None:
+            if data["email"] == '':
+                return {
+                    "msg": "Empty email is not allowed!"
+                }, HTTPStatus.BAD_REQUEST
+            elif (user.email != data["email"] and
                     db.User.exists("email", data["email"])):
                 return {
                     "msg": "User with that email already exists."
['vantage6-server/vantage6/server/resource/user.py']