Add custom exceptions, simplify generation template

tests/test_basic.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 from dataclasses import dataclass
+from typing import Optional
 
 from src.megasniff import SchemaInflatorGenerator
 
@@ -32,3 +33,14 @@ def test_circular():
     a = fn({'b': {'a': None}})
 
     return isinstance(a.b, CircB)
+
+
+def test_optional():
+    @dataclass
+    class C:
+        a: Optional[int] = None
+
+    infl = SchemaInflatorGenerator()
+    fn = infl.schema_to_generator(C)
+    c = fn({})
+    assert c.a is None