Fixed system message in chat mode. (#2343)

When system message is not in the history tokenizer inserts default
system message during applying of chat template.
Adding to the history empty system message breaks this logic which leads
to empty system message in the resulting model input.
This affects chat accuracy.

Ticket: CVS-169085

Author: popovaan

src/cpp/src/visual_language/pipeline.cpp

@@ -312,6 +312,9 @@ class VLMPipeline::VLMPipelineImpl : public VLMPipelineBase{
             m_history.clear();
         }
         m_inputs_embedder->start_chat(system_message);
+        if (system_message.empty()) {
+            return;
+        }
         m_history = {{{"role", "system"}, {"content", system_message}}};
     }
 
@@ -324,6 +327,7 @@ class VLMPipeline::VLMPipelineImpl : public VLMPipelineBase{
         m_language.get_tensor("attention_mask").set_shape({0, 0});
         // clear all chat history
         m_inputs_embedder->finish_chat();
+        m_history.clear();
     }
 
     Tokenizer get_tokenizer() const override {