Ensure BoolType maps null values correctly

Fixes #106

moor/lib/src/types/sql_types.dart

@@ -26,16 +26,24 @@ class BoolType extends SqlType<bool> {
 
   @override
   bool mapFromDatabaseResponse(response) {
+    // ignore: avoid_returning_null
+    if (response == null) return null;
     return response != 0;
   }
 
   @override
   String mapToSqlConstant(bool content) {
+    if (content == null) {
+      return 'NULL';
+    }
     return content ? '1' : '0';
   }
 
   @override
   mapToSqlVariable(bool content) {
+    if (content == null) {
+      return null;
+    }
     return content ? 1 : 0;
   }
 }

moor/test/types/sql_type_test.dart

@@ -0,0 +1,15 @@
+import 'package:moor/moor.dart' hide isNull;
+import 'package:test/test.dart';
+
+void main() {
+  test('types map null values to null', () {
+    final typeSystem = const SqlTypeSystem.withDefaults();
+
+    for (var type in typeSystem.types) {
+      expect(type.mapToSqlVariable(null), isNull,
+          reason: '$type should map null to null variables');
+      expect(type.mapFromDatabaseResponse(null), isNull,
+          reason: '$type should map null response to null value');
+    }
+  });
+}