Add test case for LLMBlock model_prompt parameter

Add a test case that ensures that passing `model_prompt` as an empty
string results in `self.model_prompt` as empty. If this parameter is
not specified (is None), `self.model_prompt` should be a non-empty
string as a result of our default behavior of picking a prompt based
on the model family.

Signed-off-by: Russell Bryant <[email protected]>

tests/test_llmblock.py

@@ -0,0 +1,57 @@
+# Standard
+from unittest.mock import MagicMock, patch
+import unittest
+
+# First Party
+from src.instructlab.sdg.llmblock import LLMBlock
+
+
+class TestLLMBlockModelPrompt(unittest.TestCase):
+    def setUp(self):
+        self.mock_ctx = MagicMock()
+        self.mock_ctx.model_family = "mixtral"
+        self.mock_ctx.model_id = "test_model"
+        self.mock_pipe = MagicMock()
+        self.config_return_value = {
+            "system": "system",
+            "introduction": "introduction",
+            "principles": "principles",
+            "examples": "examples",
+            "generation": "generation",
+        }
+
+    @patch("src.instructlab.sdg.block.Block._load_config")
+    def test_model_prompt_empty_string(self, mock_load_config):
+        mock_load_config.return_value = self.config_return_value
+        block = LLMBlock(
+            ctx=self.mock_ctx,
+            pipe=self.mock_pipe,
+            block_name="test_block",
+            config_path="",
+            output_cols=[],
+            model_prompt="",
+        )
+        self.assertEqual(
+            block.model_prompt,
+            "",
+            "model_prompt should be an empty string when explicitly set to an empty string",
+        )
+
+    @patch("src.instructlab.sdg.block.Block._load_config")
+    def test_model_prompt_none(self, mock_load_config):
+        mock_load_config.return_value = self.config_return_value
+        # Ensure that if a custom model_prompt is not specified, it defaults to setting it to
+        # something based on the model family. For this we just make sure it's not an empty string.
+        block = LLMBlock(
+            ctx=self.mock_ctx,
+            pipe=self.mock_pipe,
+            block_name="test_block",
+            config_path="",
+            output_cols=[],
+            model_prompt=None,  # Or simply omit model_prompt as it defaults to None
+        )
+        self.assertNotEqual(
+            block.model_prompt,
+            "",
+            "model_prompt should be a non-empty string when set to None",
+        )