Return 404 status code when no license is installed

closes elastic/elasticsearch#2000

Original commit: elastic/x-pack-elasticsearch@3bd4193cf8

elasticsearch/x-pack/license-plugin/src/main/java/org/elasticsearch/license/plugin/rest/RestGetLicenseAction.java

@@ -26,6 +26,7 @@ import java.util.HashMap;
 import java.util.Map;
 
 import static org.elasticsearch.rest.RestRequest.Method.GET;
+import static org.elasticsearch.rest.RestStatus.NOT_FOUND;
 import static org.elasticsearch.rest.RestStatus.OK;
 
 public class RestGetLicenseAction extends BaseRestHandler {
@@ -58,14 +59,15 @@ public class RestGetLicenseAction extends BaseRestHandler {
                         if (!request.hasParam("pretty")) {
                             builder.prettyPrint().lfAtEnd();
                         }
+                        boolean hasLicense = response.license() != null;
                         builder.startObject();
-                        if (response.license() != null) {
+                        if (hasLicense) {
                             builder.startObject("license");
                             response.license().toInnerXContent(builder, params);
                             builder.endObject();
                         }
                         builder.endObject();
-                        return new BytesRestResponse(OK, builder);
+                        return new BytesRestResponse(hasLicense ? OK : NOT_FOUND, builder);
                     }
                 });
     }

elasticsearch/x-pack/license-plugin/src/test/resources/rest-api-spec/api/license.delete.json

@@ -0,0 +1,12 @@
+{
+  "license.delete": {
+    "documentation": "https://www.elastic.co/guide/en/shield/current/license-management.html",
+    "methods": ["DELETE"],
+    "url": {
+      "path": "/_license",
+      "paths": ["/_license"],
+      "parts" : {}
+    },
+    "body": null
+  }
+}

elasticsearch/x-pack/license-plugin/src/test/resources/rest-api-spec/test/license/20_put_license.yaml

@@ -55,3 +55,14 @@
     - length: { license: 10 }
     - match: { license.uid: "893361dc-9749-4997-93cb-802e3dofh7aa" }
 
+---
+"Should throw 404 after license deletion":
+
+  - do:
+    license.delete: {}
+
+  - match: { acknowledged: true }
+
+  - do:
+      catch: missing
+      license.get: {}