Fix NPE in rolling over unknown target

Since #122905 we were throwing NPEs (i.e. 5xxs) when a rollover request
has an unknown/non-existent target. This PR restores that to it's
original behavior (which is to return a 400 - illegal argument
exception). Additionally, to avoid this from happening again, we add a
YAML test that asserts the correct exception behavior.

Author: nielsbauman

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/indices.rollover/10_basic.yml

@@ -183,3 +183,11 @@
             min_age: "0s"
             min_docs: 1
   - match: { error.reason: "Validation Failed: 1: at least one max_* rollover condition must be set when using min_* conditions;" }
+
+---
+"Rolling over an unknown target should return 404":
+  - do:
+      catch: bad_request
+      indices.rollover:
+        alias: "non_existent"
+  - match: {error.reason: "rollover target [non_existent] does not exist"}

server/src/main/java/org/elasticsearch/action/admin/indices/rollover/TransportRolloverAction.java

@@ -164,7 +164,7 @@ protected ClusterBlockException checkBlock(RolloverRequest request, ClusterState
         ResolvedExpression resolvedRolloverTarget = SelectorResolver.parseExpression(request.getRolloverTarget(), request.indicesOptions());
         final IndexAbstraction indexAbstraction = projectMetadata.getIndicesLookup().get(resolvedRolloverTarget.resource());
         final String[] indicesToCheck;
-        if (indexAbstraction.getType().equals(IndexAbstraction.Type.DATA_STREAM)) {
+        if (indexAbstraction != null && indexAbstraction.getType().equals(IndexAbstraction.Type.DATA_STREAM)) {
             DataStream dataStream = (DataStream) indexAbstraction;
             boolean targetFailureStore = resolvedRolloverTarget.selector() != null
                 && resolvedRolloverTarget.selector().shouldIncludeFailures();