fix: raise EncodeError when encoding None with explicit non-NULL type

encode_value(None, ValueType.INTEGER) previously silently returned NULL
bytes with NULL type code, ignoring the caller's explicit type request.
The early-return for None at the top of encode_value fired before the
value_type parameter was inspected.

Now raises EncodeError when value is None and an explicit type other
than ValueType.NULL is provided. encode_value(None) and
encode_value(None, ValueType.NULL) continue to work as before.

Closes #094

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antoineleclair

src/dqlitewire/types.py

@@ -172,6 +172,11 @@ def encode_value(value: Any, value_type: ValueType | None = None) -> tuple[bytes
     Returns (encoded_data, value_type).
     """
     if value is None:
+        if value_type is not None and value_type != ValueType.NULL:
+            raise EncodeError(
+                f"Cannot encode None with explicit type {value_type.name}. "
+                f"Pass value_type=ValueType.NULL or omit value_type."
+            )
         return b"\x00" * 8, ValueType.NULL
 
     if value_type is None:

tests/test_types.py

@@ -736,6 +736,28 @@ def test_encode_value_null_type_with_string_raises(self) -> None:
         with pytest.raises(EncodeError, match="Cannot encode non-None value"):
             encode_value("hello", ValueType.NULL)
 
+    def test_encode_value_none_with_explicit_integer_raises(self) -> None:
+        """None with explicit ValueType.INTEGER should raise EncodeError."""
+        with pytest.raises(EncodeError, match="Cannot encode None with explicit type"):
+            encode_value(None, ValueType.INTEGER)
+
+    def test_encode_value_none_with_explicit_text_raises(self) -> None:
+        """None with explicit ValueType.TEXT should raise EncodeError."""
+        with pytest.raises(EncodeError, match="Cannot encode None with explicit type"):
+            encode_value(None, ValueType.TEXT)
+
+    def test_encode_value_none_with_explicit_null_ok(self) -> None:
+        """None with explicit ValueType.NULL should succeed."""
+        encoded, vtype = encode_value(None, ValueType.NULL)
+        assert vtype == ValueType.NULL
+        assert encoded == b"\x00" * 8
+
+    def test_encode_value_none_without_type_ok(self) -> None:
+        """None without explicit type should succeed (infer NULL)."""
+        encoded, vtype = encode_value(None)
+        assert vtype == ValueType.NULL
+        assert encoded == b"\x00" * 8
+
     def test_parse_iso8601_rejects_garbage(self) -> None:
         """Unparseable ISO 8601 string should raise DecodeError."""
         from dqlitewire.types import _parse_iso8601