Verify not empty rendered chat template (#1574)

Co-authored-by: Ilya Lavrenov <ilya.lavrenov@intel.com>

Author: yatarkan

src/cpp/src/tokenizer.cpp

@@ -575,13 +575,18 @@ class Tokenizer::TokenizerImpl {
             {"slice", slice_callable},
         };
 
+        std::string result;
         try {
-            return tpl.RenderAsString(params).value();
+            result = tpl.RenderAsString(params).value();
         } catch (const std::exception& error) {
             OPENVINO_THROW("Chat template for the current model is not supported by Jinja2Cpp. "
                            "Please apply template manually to your prompt before calling generate. "
                            "For example: <start_of_turn>user{user_prompt}<end_of_turn><start_of_turn>model");
         }
+        OPENVINO_ASSERT(!result.empty(), "Applied chat template resulted in an empty string. "
+                                         "Please check the chat template or apply template manually to your prompt before calling generate."
+                                         "For example: <start_of_turn>user{user_prompt}<end_of_turn><start_of_turn>model");
+        return result;
     }
 
     void set_chat_template(const std::string& chat_template) {

tests/python_tests/test_tokenizer.py

@@ -181,6 +181,12 @@ def test_apply_chat_template(model_tmp_path, chat_config: Tuple[str, Dict]):
         print(f'ov_genai out: {ov_full_history_str}')
     assert ov_full_history_str == hf_full_history_str
 
+    # Test throwing exception for empty rendered chat template
+    # Example: Qwen2-VL chat template
+    chat_template_for_empty_output = "{% if messages is string %}{{ messages }}{% else %}{% for content in messages %}{% if content['type'] == 'image' or 'image' in content or 'image_url' in content %}<|vision_start|><|image_pad|><|vision_end|>{% elif content['type'] == 'video' or 'video' in content %}<|vision_start|><|video_pad|><|vision_end|>{% elif 'text' in content %}{{ content['text'] }}{% endif %}{% endfor %}{% endif %}"
+    with pytest.raises(Exception):
+        ov_tokenizer.apply_chat_template(conversation, chat_template=chat_template_for_empty_output)
+
 
 @pytest.mark.precommit
 @pytest.mark.nightly