fix param serialization when system message is None

Author: bburdiliak

anthropic-client/src/main/scala/io/cequence/openaiscala/anthropic/service/impl/AnthropicServiceImpl.scala

@@ -112,7 +112,7 @@ private[service] trait AnthropicServiceImpl extends Anthropic {
     jsonBodyParams(
       Param.messages -> Some(messageJsons),
       Param.model -> Some(settings.model),
-      Param.system -> Some(systemJson),
+      Param.system -> system.map(_ => systemJson),
       Param.max_tokens -> Some(settings.max_tokens),
       Param.metadata -> { if (settings.metadata.isEmpty) None else Some(settings.metadata) },
       Param.stop_sequences -> {

openai-examples/src/main/scala/io/cequence/openaiscala/examples/nonopenai/AnthropicCreateMessageWithPdf.scala

@@ -1,7 +1,7 @@
 package io.cequence.openaiscala.examples.nonopenai
 
 import io.cequence.openaiscala.anthropic.domain.Content.ContentBlock.{MediaBlock, TextBlock}
-import io.cequence.openaiscala.anthropic.domain.Content.ContentBlockBase
+import io.cequence.openaiscala.anthropic.domain.Content.{ContentBlockBase, SingleString}
 import io.cequence.openaiscala.anthropic.domain.Message
 import io.cequence.openaiscala.anthropic.domain.Message.UserMessageContent
 import io.cequence.openaiscala.anthropic.domain.response.CreateMessageResponse
@@ -37,7 +37,6 @@ object AnthropicCreateMessageWithPdf extends ExampleBase[AnthropicService] {
     service
       .createMessage(
         messages,
-        None,
         settings = AnthropicCreateMessageSettings(
           model =
             NonOpenAIModelId.claude_3_5_sonnet_20241022, // claude-3-5-sonnet-20241022 supports PDF (beta)