Fixing empty input variable crashing PromptTemplate validations (#14314)

- Fixes `input_variables=[""]` crashing validations with a template
`"{}"`
- Uses `__cause__` for proper `Exception` chaining in
`check_valid_template`

libs/core/langchain_core/prompts/string.py

@@ -106,20 +106,20 @@ def check_valid_template(
     Raises:
         ValueError: If the template format is not supported.
     """
-    if template_format not in DEFAULT_FORMATTER_MAPPING:
-        valid_formats = list(DEFAULT_FORMATTER_MAPPING)
-        raise ValueError(
-            f"Invalid template format. Got `{template_format}`;"
-            f" should be one of {valid_formats}"
-        )
     try:
         validator_func = DEFAULT_VALIDATOR_MAPPING[template_format]
-        validator_func(template, input_variables)
-    except KeyError as e:
+    except KeyError as exc:
         raise ValueError(
-            "Invalid prompt schema; check for mismatched or missing input parameters. "
-            + str(e)
-        )
+            f"Invalid template format {template_format!r}, should be one of"
+            f" {list(DEFAULT_FORMATTER_MAPPING)}."
+        ) from exc
+    try:
+        validator_func(template, input_variables)
+    except (KeyError, IndexError) as exc:
+        raise ValueError(
+            "Invalid prompt schema; check for mismatched or missing input parameters"
+            f" from {input_variables}."
+        ) from exc
 
 
 def get_template_variables(template: str, template_format: str) -> List[str]:

libs/core/tests/unit_tests/prompts/test_prompt.py

@@ -47,6 +47,12 @@ def test_prompt_missing_input_variables() -> None:
     ).input_variables == ["foo"]
 
 
+def test_prompt_empty_input_variable() -> None:
+    """Test error is raised when empty string input variable."""
+    with pytest.raises(ValueError):
+        PromptTemplate(input_variables=[""], template="{}", validate_template=True)
+
+
 def test_prompt_extra_input_variables() -> None:
     """Test error is raised when there are too many input variables."""
     template = "This is a {foo} test."