[placement] Require at least one allocation when PUT

When doing a PUT to /allocations/{consumer_uuid} if the list of
allocations is empty there is a 500 error coming from an uncaught
IndexError.

Adjust the schema to require at least one allocation. If no allocations
are provided in the list, the response will be a 400.

This doesn't require a microversion because it is changing a 500 to
a useful response.

Change-Id: I361fa1e534d7b20c985558e272b672b84c6fe439
Closes-Bug: #1708260

nova/api/openstack/placement/handlers/allocation.py

@@ -34,6 +34,7 @@ ALLOCATION_SCHEMA = {
     "properties": {
         "allocations": {
             "type": "array",
+            "minItems": 1,
             "items": {
                 "type": "object",
                 "properties": {

nova/tests/functional/api/openstack/placement/gabbits/allocations.yaml

@@ -54,6 +54,16 @@ tests:
   response_json_paths:
       $.errors[0].title: Bad Request
 
+- name: put an allocation empty list
+  PUT: /allocations/599ffd2d-526a-4b2e-8683-f13ad25f9958
+  request_headers:
+      content-type: application/json
+  data:
+      allocations: []
+  status: 400
+  response_strings:
+      - "Failed validating 'minItems'"
+
 - name: put an allocation violate schema
   PUT: /allocations/599ffd2d-526a-4b2e-8683-f13ad25f9958
   request_headers: