Return appropriate error on null dims update instead of npe (#125716)

Calling `Object::toString` was trying to call `null.toString()`, really
it should have been `Objects::toString`, which accepts `null`.

closes: https://github.com/elastic/elasticsearch/issues/125713

docs/changelog/125716.yaml

@@ -0,0 +1,5 @@
+pr: 125716
+summary: Return appropriate error on null dims update instead of npe
+area: Vector Search
+type: bug
+issues: []

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/40_knn_search.yml

@@ -605,3 +605,28 @@ setup:
   - match: { hits.hits.0._score: $knn_score0 }
   - match: { hits.hits.1._score: $knn_score1 }
   - match: { hits.hits.2._score: $knn_score2 }
+---
+"Updating dim to null is not allowed":
+  - requires:
+      cluster_features: "mapper.npe_on_dims_update_fix"
+      reason: "dims update fix"
+  - do:
+      indices.create:
+        index: test_index
+
+  - do:
+      indices.put_mapping:
+        index: test_index
+        body:
+          properties:
+            embedding:
+              type: dense_vector
+              dims: 4
+  - do:
+      catch: bad_request
+      indices.put_mapping:
+        index: test_index
+        body:
+          properties:
+            embedding:
+              type: dense_vector

server/src/main/java/org/elasticsearch/index/mapper/MapperFeatures.java

@@ -39,6 +39,7 @@ public class MapperFeatures implements FeatureSpecification {
     static final NodeFeature UKNOWN_FIELD_MAPPING_UPDATE_ERROR_MESSAGE = new NodeFeature(
         "mapper.unknown_field_mapping_update_error_message"
     );
+    static final NodeFeature NPE_ON_DIMS_UPDATE_FIX = new NodeFeature("mapper.npe_on_dims_update_fix");
 
     @Override
     public Set<NodeFeature> getTestFeatures() {
@@ -64,6 +65,7 @@ public class MapperFeatures implements FeatureSpecification {
             DOC_VALUES_SKIPPER,
             RESCORE_VECTOR_QUANTIZED_VECTOR_MAPPING,
             DateFieldMapper.INVALID_DATE_FIX,
+            NPE_ON_DIMS_UPDATE_FIX,
             RESCORE_ZERO_VECTOR_QUANTIZED_VECTOR_MAPPING
         );
     }

server/src/main/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldMapper.java

@@ -154,7 +154,7 @@ public class DenseVectorFieldMapper extends FieldMapper {
             }
 
             return XContentMapValues.nodeIntegerValue(o);
-        }, m -> toType(m).fieldType().dims, XContentBuilder::field, Object::toString).setSerializerCheck((id, ic, v) -> v != null)
+        }, m -> toType(m).fieldType().dims, XContentBuilder::field, Objects::toString).setSerializerCheck((id, ic, v) -> v != null)
             .setMergeValidator((previous, current, c) -> previous == null || Objects.equals(previous, current))
             .addValidator(dims -> {
                 if (dims == null) {

x-pack/plugin/rank-vectors/src/main/java/org/elasticsearch/xpack/rank/vectors/mapper/RankVectorsFieldMapper.java

@@ -89,7 +89,7 @@ public class RankVectorsFieldMapper extends FieldMapper {
             }
 
             return XContentMapValues.nodeIntegerValue(o);
-        }, m -> toType(m).fieldType().dims, XContentBuilder::field, Object::toString).setSerializerCheck((id, ic, v) -> v != null)
+        }, m -> toType(m).fieldType().dims, XContentBuilder::field, Objects::toString).setSerializerCheck((id, ic, v) -> v != null)
             .setMergeValidator((previous, current, c) -> previous == null || Objects.equals(previous, current))
             .addValidator(dims -> {
                 if (dims == null) {

x-pack/plugin/src/yamlRestTest/resources/rest-api-spec/test/rank_vectors/rank_vectors.yml

@@ -135,3 +135,28 @@ setup:
         id: "1"
         body:
           vector1: [[2, -1, 1], [[2, -1, 1]]]
+---
+"Updating dim to null is not allowed":
+  - requires:
+      cluster_features: "mapper.npe_on_dims_update_fix"
+      reason: "dims update fix"
+  - do:
+      indices.create:
+        index: test_index
+
+  - do:
+      indices.put_mapping:
+        index: test_index
+        body:
+          properties:
+            embedding:
+              type: rank_vectors
+              dims: 4
+  - do:
+      catch: bad_request
+      indices.put_mapping:
+        index: test_index
+        body:
+          properties:
+            embedding:
+              type: rank_vectors