Throw better exception on wrong dynamic_templates syntax (elastic#51783)

Currently, a mappings update request, where dynamic_mappings is an object
instead of an array, results in a http response with a 500 code. This PR checks
for this condition and throws a MapperParsingException like we do for other
malformed mapping cases.

Closes elastic#51486

Author: feifeiiiiiiiiiii

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

@@ -168,6 +168,9 @@ protected boolean processField(RootObjectMapper.Builder builder, String fieldNam
                       }
                   ]
                 */
+                if ((fieldNode instanceof List) == false) {
+                    throw new MapperParsingException("Dynamic template syntax error. An array of named objects is expected.");
+                }
                 List<?> tmplNodes = (List<?>) fieldNode;
                 List<DynamicTemplate> templates = new ArrayList<>();
                 for (Object tmplNode : tmplNodes) {

server/src/test/java/org/elasticsearch/index/mapper/RootObjectMapperTests.java

@@ -185,4 +185,19 @@ public void testIllegalFormatField() throws Exception {
             assertEquals("Invalid format: [[test_format]]: expected string value", e.getMessage());
         }
     }
+
+    public void testIllegalDynamicTemplates() throws Exception {
+        String mapping = Strings.toString(XContentFactory.jsonBuilder()
+                .startObject()
+                .startObject("type")
+                    .startObject("dynamic_templates")
+                    .endObject()
+                .endObject()
+            .endObject());
+
+        DocumentMapperParser parser = createIndex("test").mapperService().documentMapperParser();
+        MapperParsingException e = expectThrows(MapperParsingException.class,
+                    () -> parser.parse("type", new CompressedXContent(mapping)));
+            assertEquals("Dynamic template syntax error. An array of named objects is expected.", e.getMessage());
+    }
 }