NPE trying to set autoscaling policy for existing collection

Steps to reproduce:

• create a collection without collection-specific policy, eg. test
• define a collection-specific policy policy1:

  POST http://localhost:8983/solr/admin/autoscaling
  {
  "set-policy": 
    {
  	"policy1" :[
  		{"replica": "<2", "shard": "#EACH", "node": "#ANY"}
  	]
    }
  }
  

• try to modify the collection to use this policy

  http://localhost:8983/solr/admin/collections?action=MODIFYCOLLECTION&collection=test&policy=policy1
  

  A NullPointerException is thrown due to the previous value of the "policy" property being absent:

  2019-02-14 18:48:17.007 ERROR (OverseerThreadFactory-9-thread-5-processing-n:192.168.0.69:8983_solr) [c:test   ] o.a.s.c.a.c.OverseerCollectionMessageHandler Collection: test operation: modifycollection failed:java.lang.NullPointerException
          at org.apache.solr.cloud.api.collections.OverseerCollectionMessageHandler.modifyCollection(OverseerCollectionMessageHandler.java:687)
          at org.apache.solr.cloud.api.collections.OverseerCollectionMessageHandler.processMessage(OverseerCollectionMessageHandler.java:292)
          at org.apache.solr.cloud.OverseerTaskProcessor$Runner.run(OverseerTaskProcessor.java:496)
          at org.apache.solr.common.util.ExecutorUtil$MDCAwareThreadPoolExecutor.lambda$execute$0(ExecutorUtil.java:209)
          at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
          at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
          at java.lang.Thread.run(Thread.java:748)