fix(data_model): silence spurious "Cluster cannot be NULL" error during dynamic endpoint init

When using the esp_matter data model (CONFIG_ESP_MATTER_ENABLE_DATA_MODEL=y),
attribute::get(endpoint_id, cluster_id, attribute_id) is called during endpoint
registration via emberAfExternalAttributeReadCallback. If the cluster doesn't
exist on the endpoint, the lookup returns NULL, which is then passed to the
two-argument get(cluster_t*, attribute_id) overload that logs at error level.

Add a NULL guard in the three-argument overload to return nullptr early,
consistent with how command::get(endpoint_id, cluster_id, command_id) already
handles this case.

Fixes #1692

Author: cleishm

components/esp_matter/data_model/esp_matter_data_model.cpp

@@ -222,7 +222,7 @@ static void report_parts_list_change_internal(endpoint_t *endpoint)
         parent_endpoint_id = endpoint::get_parent_endpoint_id(endpoint::get(parent_endpoint_id));
     }
     MatterReportingAttributeChangeCallback(/* endpoint = */ 0, chip::app::Clusters::Descriptor::Id,
-                                           chip::app::Clusters::Descriptor::Attributes::PartsList::Id);
+                                                            chip::app::Clusters::Descriptor::Attributes::PartsList::Id);
 }
 
 esp_err_t disable(endpoint_t *endpoint)
@@ -696,6 +696,9 @@ attribute_t *get(cluster_t *cluster, uint32_t attribute_id)
 attribute_t *get(uint16_t endpoint_id, uint32_t cluster_id, uint32_t attribute_id)
 {
     cluster_t *cluster = cluster::get(endpoint_id, cluster_id);
+    if (!cluster) {
+        return nullptr;
+    }
     return get(cluster, attribute_id);
 }