Fix tedge-agent panic on entity registration

Mixing entity registrations over MQTT and HTTP:

```
tedge mqtt pub te/device/a/service/c '{"@parent":"device/a//","@type":"service"}'
tedge mqtt pub te/device/ac// '{"@parent":"device/a//","@type":"child-device"}'
curl http://localhost:8000/tedge/entity-store/v1/entities -v -H "Content-Type: application/json" -d '{"@topic-id": "device/a/service/c", "@type": "service", "@parent": "device/a//"}'
curl http://localhost:8000/tedge/entity-store/v1/entities -v -H "Content-Type: application/json" -d '{"@topic-id": "device/a//", "@type": "child-device", "@parent": "device/main//"}'
```

was crashing the agent:

```
thread 'tokio-runtime-worker' panicked at crates/core/tedge_api/src/store/pending_entity_store.rs:96:73:
called `Option::unwrap()` on a `None` value
```

Signed-off-by: Didier Wenzek <[email protected]>

Author: didier-wenzek

crates/core/tedge_api/src/store/pending_entity_store.rs

@@ -85,18 +85,21 @@ impl PendingEntityStore {
         }
     }
 
-    /// Returns all the children and their children recursively for a given parent
+    /// Recursively removes from the pending entity cache the children of a freshly registered device.
+    ///
+    /// Returns the list of devices which registration is no more pending.
     pub fn take_cached_child_entities_data(
         &mut self,
         entity_tid: &EntityTopicId,
     ) -> Vec<RegisteredEntityData> {
         let mut children = vec![];
         if let Some(direct_children) = self.orphans.remove(entity_tid) {
             for child in direct_children {
-                let pending_entity_cache = self.entities.remove(&child).unwrap();
-                let pending_entity_data = self.registered_data_from_cache(pending_entity_cache);
-                children.push(pending_entity_data);
-                children.append(&mut self.take_cached_child_entities_data(&child));
+                if let Some(pending_entity_cache) = self.entities.remove(&child) {
+                    let pending_entity_data = self.registered_data_from_cache(pending_entity_cache);
+                    children.push(pending_entity_data);
+                    children.append(&mut self.take_cached_child_entities_data(&child));
+                }
             }
         }
         children