[Rest Api Compatibility] Do not return _doc for empty mappings in template (elastic#75448)

Previously the compatibility layer was always returning an _doc in mappings for get
template.
This commit does not return _doc in empty mappings.
Returning just {} empty object (v7 and v8 behaviour)

also moving term lookups tests which are already fixed (relates elastic#74544)

relates elastic#70966
relates main meta issue elastic#51816
relates types removal meta elastic#54160

Author: pgomulka

rest-api-spec/build.gradle

@@ -78,8 +78,9 @@ def v7compatibilityNotSupportedTests = {
 
           'indices.create/10_basic/Create index without soft deletes', //Make soft-deletes mandatory in 8.0 #51122 - settings changes are note supported in Rest Api compatibility
 
-
           'field_caps/30_filter/Field caps with index filter', //behaviour change after #63692 4digits dates are parsed as epoch and in quotes as year
+
+          'indices.forcemerge/10_basic/Check deprecation warning when incompatible only_expunge_deletes and max_num_segments values are both set', //#44761 bug fix
   ]
 }
 tasks.named("yamlRestCompatTest").configure {
@@ -91,12 +92,7 @@ tasks.named("yamlRestCompatTest").configure {
     'cluster.voting_config_exclusions/10_basic/Throw exception when adding voting config exclusion and specifying both node_ids and node_names',
     'cluster.voting_config_exclusions/10_basic/Throw exception when adding voting config exclusion without specifying nodes',
     'indices.flush/10_basic/Index synced flush rest test',
-    'indices.forcemerge/10_basic/Check deprecation warning when incompatible only_expunge_deletes and max_num_segments values are both set',
-    // not fixing this in #70966
-    'indices.put_template/11_basic_with_types/Put template with empty mappings',
     'search.aggregation/200_top_hits_metric/top_hits aggregation with sequence numbers',
-    'search.aggregation/51_filter_with_types/Filter aggs with terms lookup and ensure it\'s cached',
-    'search/150_rewrite_on_coordinator/Ensure that we fetch the document only once', //terms_lookup
     'search/310_match_bool_prefix/multi_match multiple fields with cutoff_frequency throws exception', //cutoff_frequency
     'search/340_type_query/type query', // type_query - probably should behave like match_all
   ] + v7compatibilityNotSupportedTests())

server/src/main/java/org/elasticsearch/cluster/metadata/IndexTemplateMetadata.java

@@ -27,6 +27,7 @@
 import org.elasticsearch.common.xcontent.XContentHelper;
 import org.elasticsearch.common.xcontent.XContentParser;
 import org.elasticsearch.common.xcontent.json.JsonXContent;
+import org.elasticsearch.index.mapper.MapperService;
 
 import java.io.IOException;
 import java.io.UncheckedIOException;
@@ -391,6 +392,8 @@ private static void toInnerXContent(IndexTemplateMetadata indexTemplateMetadata,
                 Map<String, Object> documentMapping = XContentHelper.convertToMap(m.uncompressed(), true).v2();
                 if (includeTypeName == false) {
                     documentMapping = reduceMapping(documentMapping);
+                } else {
+                    documentMapping = reduceEmptyMapping(documentMapping);
                 }
                 builder.field("mappings");
                 builder.map(documentMapping);
@@ -405,6 +408,16 @@ private static void toInnerXContent(IndexTemplateMetadata indexTemplateMetadata,
             builder.endObject();
         }
 
+        @SuppressWarnings("unchecked")
+        private static Map<String, Object> reduceEmptyMapping(Map<String, Object> mapping) {
+            if(mapping.keySet().size() == 1 && mapping.containsKey(MapperService.SINGLE_MAPPING_NAME) &&
+                ((Map<String, Object>)mapping.get(MapperService.SINGLE_MAPPING_NAME)).size() == 0){
+                return (Map<String, Object>) mapping.values().iterator().next();
+            } else {
+                return mapping;
+            }
+        }
+
         @SuppressWarnings("unchecked")
         private static Map<String, Object> reduceMapping(Map<String, Object> mapping) {
             assert mapping.keySet().size() == 1 : mapping.keySet();