fix[ux]: fix empty hints in error messages (#4351)

remove empty hint message from exceptions. this regression was
introduced in 03095ce. the root cause of the issue, however,
was that `hint=""` could be constructed in the first place. this commit
fixes the `get_levenshtein_error_suggestions` helper so that it returns
`None` on failure to find a suggestion rather than the empty string
`""`.

---------

Co-authored-by: Charles Cooper <[email protected]>

tests/functional/syntax/exceptions/test_undeclared_definition.py

@@ -66,5 +66,6 @@ def foo():
 
 @pytest.mark.parametrize("bad_code", fail_list)
 def test_undeclared_def_exception(bad_code):
-    with pytest.raises(UndeclaredDefinition):
+    with pytest.raises(UndeclaredDefinition) as e:
         compiler.compile_code(bad_code)
+    assert "(hint: )" not in str(e.value)

tests/functional/syntax/exceptions/test_unknown_type.py

@@ -0,0 +1,15 @@
+import pytest
+
+from vyper import compiler
+from vyper.exceptions import UnknownType
+
+
+def test_unknown_type_exception():
+    code = """
+@internal
+def foobar(token: IERC20):
+    pass
+    """
+    with pytest.raises(UnknownType) as e:
+        compiler.compile_code(code)
+    assert "(hint: )" not in str(e.value)

vyper/semantics/analysis/levenshtein_utils.py

@@ -1,4 +1,4 @@
-from typing import Any, Callable
+from typing import Any, Callable, Optional
 
 
 def levenshtein_norm(source: str, target: str) -> float:
@@ -79,7 +79,7 @@ def get_levenshtein_error_suggestions(*args, **kwargs) -> Callable:
 
 def _get_levenshtein_error_suggestions(
     key: str, namespace: dict[str, Any], threshold: float
-) -> str:
+) -> Optional[str]:
     """
     Generate an error message snippet for the suggested closest values in the provided namespace
     with the shortest normalized Levenshtein distance from the given key if that distance
@@ -100,11 +100,11 @@ def _get_levenshtein_error_suggestions(
     """
 
     if key is None or key == "":
-        return ""
+        return None
 
     distances = sorted([(i, levenshtein_norm(key, i)) for i in namespace], key=lambda k: k[1])
     if len(distances) > 0 and distances[0][1] <= threshold:
         if len(distances) > 1 and distances[1][1] <= threshold:
             return f"Did you mean '{distances[0][0]}', or maybe '{distances[1][0]}'?"
         return f"Did you mean '{distances[0][0]}'?"
-    return ""
+    return None