Allow for None as a literal placeholder value

This is acceptable alongside boolean values as I don't think it is
possible to SQL Inject with a literal `None` and is useful.

Author: pgjones

src/sql_tstring/__init__.py

@@ -145,6 +145,8 @@ def _safely_convert_placeholder_value(
     if case_insensitive is None:
         case_insensitive = set()
 
+    if value is None:
+        return "NULL"
     if isinstance(value, bool):
         return str(value)
     if allow_numeric and isinstance(value, Number):

tests/test_identifiers.py

@@ -45,6 +45,13 @@ def test_numeric_invalid() -> None:
         sql("SELECT {a}", locals())
 
 
+def test_none() -> None:
+    a = None
+    query, values = sql("SELECT {a}", locals())
+    assert query == "SELECT NULL"
+    assert values == []
+
+
 def test_partition_by() -> None:
     a = RewritingValue.ABSENT
     b = "x"