Fix NPE if field caps request has a field that exists not in all indices (#24504)

If a field caps request contains a field name that doesn't exist in all indices
the response will be partial and we hide an NPE. The NPE is now fixed but we still
have the problem that we don't pass on errors on the shard level to the user. This will
be fixed in a followup.

core/src/main/java/org/elasticsearch/action/fieldcaps/TransportFieldCapabilitiesIndexAction.java

@@ -80,8 +80,10 @@ public class TransportFieldCapabilitiesIndexAction extends TransportSingleShardA
         Map<String, FieldCapabilities> responseMap = new HashMap<>();
         for (String field : fieldNames) {
             MappedFieldType ft = mapperService.fullName(field);
-            FieldCapabilities fieldCap = new FieldCapabilities(field, ft.typeName(), ft.isSearchable(), ft.isAggregatable());
+            if (ft != null) {
-            responseMap.put(field, fieldCap);
+                FieldCapabilities fieldCap = new FieldCapabilities(field, ft.typeName(), ft.isSearchable(), ft.isAggregatable());
+                responseMap.put(field, fieldCap);
+            }
         }
         return new FieldCapabilitiesIndexResponse(shardId.getIndexName(), responseMap);
     }

rest-api-spec/src/main/resources/rest-api-spec/test/field_caps/10_basic.yaml

@@ -165,3 +165,45 @@ setup:
   - match: {fields.number.long.indices:                 ["test3"]}
   - is_false: fields.number.long.non_searchable_indices
   - is_false: fields.number.long.non_aggregatable_indices
+
+---
+"Mix in non-existing field field caps":
+  - skip:
+      version: " - 5.4.0"
+      reason:  this bug has been fixed in 5.4.0
+
+  - do:
+      field_caps:
+        index: 'test1,test2,test3'
+        fields: [text, keyword, no_such_field, number, geo]
+
+  - match: {fields.text.text.searchable:                true}
+  - match: {fields.text.text.aggregatable:              false}
+  - is_false: fields.text.text.indices
+  - is_false: fields.text.text.non_searchable_indices
+  - is_false: fields.text.text.non_aggregatable_indices
+  - match: {fields.keyword.keyword.searchable:          true}
+  - match: {fields.keyword.keyword.aggregatable:        true}
+  - is_false: fields.text.keyword.indices
+  - is_false: fields.text.keyword.non_searchable_indices
+  - is_false: fields.text.keyword.non_aggregatable_indices
+  - match: {fields.number.double.searchable:            true}
+  - match: {fields.number.double.aggregatable:          true}
+  - match: {fields.number.double.indices:               ["test1", "test2"]}
+  - is_false: fields.number.double.non_searchable_indices
+  - is_false: fields.number.double.non_aggregatable_indices
+  - match: {fields.number.long.searchable:              true}
+  - match: {fields.number.long.aggregatable:            true}
+  - match: {fields.number.long.indices:                 ["test3"]}
+  - is_false: fields.number.long.non_searchable_indices
+  - is_false: fields.number.long.non_aggregatable_indices
+  - match: {fields.geo.geo_point.searchable:            true}
+  - match: {fields.geo.geo_point.aggregatable:          true}
+  - match: {fields.geo.geo_point.indices:               ["test1", "test2"]}
+  - is_false: fields.geo.geo_point.non_searchable_indices
+  - is_false: fields.geo.geo_point.non_aggregatable_indices
+  - match: {fields.geo.keyword.searchable:              true}
+  - match: {fields.geo.keyword.aggregatable:            true}
+  - match: {fields.geo.keyword.indices:                 ["test3"]}
+  - is_false: fields.geo.keyword.non_searchable_indices
+  - is_false: fields.geo.keyword.on_aggregatable_indices