fix: only require at least 1 field to be present in a collection update

The collection update endpoint only requires that at least one field is
present - "fields" is not strictly required (e.g. for metadata updates).

Author: jackwilsdon

openapi.yml

@@ -2479,13 +2479,12 @@ components:
           description: >
             Optional details about the collection, e.g., when it was created, who created it etc.
     CollectionUpdateSchema:
-      required:
-        - fields
       type: object
       properties:
         fields:
           type: array
           description: A list of fields for querying, filtering and faceting
+          minItems: 1
           example:
             - name: company_name
               type: string