[TEST] don't check exact size in mapper-size yaml test

Rather test that the size is present and greather than zero. The actual size depends on the content-type, which is randomized.

plugins/mapper-size/src/test/resources/rest-api-spec/test/mapper_size/10_basic.yaml

@@ -28,5 +28,5 @@
             id: 1
             stored_fields: "_size"
 
-    - match: { _size: 13 }
+    - gt: { _size: 0 }