Allow for null/empty content field

Author: Max Hniebergall

server/src/main/java/org/elasticsearch/inference/UnifiedCompletionRequest.java

@@ -122,7 +122,7 @@ public record Message(Content content, String role, @Nullable String name, @Null
         );
 
         static {
-            PARSER.declareField(constructorArg(), (p, c) -> parseContent(p), new ParseField("content"), ObjectParser.ValueType.VALUE_ARRAY);
+            PARSER.declareField(optionalConstructorArg(), (p, c) -> parseContent(p), new ParseField("content"), ObjectParser.ValueType.VALUE_ARRAY);
             PARSER.declareString(constructorArg(), new ParseField("role"));
             PARSER.declareString(optionalConstructorArg(), new ParseField("name"));
             PARSER.declareString(optionalConstructorArg(), new ParseField("tool_call_id"));

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/external/request/openai/OpenAiUnifiedChatCompletionRequestEntity.java

@@ -78,6 +78,9 @@ public XContentBuilder toXContent(XContentBuilder builder, Params params) throws
                             }
                             builder.endArray();
                         }
+                        case null -> {
+                            // do nothing
+                        }
                     }
 
                     builder.field(ROLE_FIELD, message.role());