Avoid update with empty entities when using multi entity plugin or explicitAttrs=true

When using multi entity plugin together with expressionAttrs, a particular situation could happen when the base entity does not have any attribute within the measure.

As an example, provisioning a group like this one:

{
 "commands": [],
 "type": "GPS",
 "lazy": [],
 "active": [
     {
         "name": "explicit",
         "type": "number",
         "entity_name": "SO5",
         "object_id": "x"
     },
     {
         "name": "explicit",
         "type": "number",
         "entity_name": "SO6",
         "object_id": "y"
     }
 ],
 "explicitAttrs": true
}

And sending a measure like this one to a device gps1:

[
 {
     "name": "x",
     "type": "Number",
     "value": 52
 },
 {
     "name": "y",
     "type": "Number",
     "value": 13
 },
 {
     "name": "z",
     "type": "Number",
     "value": 12
 }
]

Would result on a update operation against CB like this one:

{
   "actionType": "update",
   "entities": [
     {
       "id": "gps1",
       "type": "GPS"
     },
     {
       "explicit": {
         "type": "number",
         "value": 52
       },
       "type": "GPS",
       "id": "SO5"
     },
     {
       "explicit": {
         "type": "number",
         "value": 13
       },
       "type": "GPS",
       "id": "SO6"
     }
   ]
 }

In which the entity gps1 is updated without having any attribute.

Makes this sense? Should this entity be removed from update list?

According to @fgalan , this does not have a huge impact on CB.

This is related with test/unit/ngsiv2/expressions/jexlBasedTransformations-test.js changes added in PR #1157

Start with test/unit/ngsiv2/expressions/jexlBasedTransformations-test.js and the changes from PR #1157, then trace how the multi-entity and expressionAttrs inputs produce the update payload. Done means the expected handling of a base entity with no attributes is decided and covered by a regression test.