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/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."