Fix NPE when parsing a real type

Fixes #32

moor/lib/src/types/sql_types.dart

@@ -116,10 +116,15 @@ class RealType extends SqlType<double> {
   const RealType();
 
   @override
-  double mapFromDatabaseResponse(response) => (response as num).toDouble();
+  double mapFromDatabaseResponse(response) => (response as num)?.toDouble();
 
   @override
-  String mapToSqlConstant(num content) => content.toString();
+  String mapToSqlConstant(num content) {
+    if (content == null) {
+      return 'NULL';
+    }
+    return content.toString();
+  }
 
   @override
   mapToSqlVariable(num content) => content;

moor/test/types/real_type_test.dart

@@ -0,0 +1,29 @@
+import 'package:moor/moor.dart' as moor;
+import 'package:test_api/test_api.dart';
+
+void main() {
+  final type = const moor.RealType();
+
+  group('RealType', () {
+    test('can be read from floating point values returned by sql', () {
+      expect(type.mapFromDatabaseResponse(3.1234), 3.1234);
+    });
+
+    test('can read null value from sql', () {
+      expect(type.mapFromDatabaseResponse(null), isNull);
+    });
+
+    test('can be mapped to sql constants', () {
+      expect(type.mapToSqlConstant(1.123), '1.123');
+    });
+
+    test('can be mapped to variables', () {
+      expect(type.mapToSqlVariable(1.123), 1.123);
+    });
+
+    test('map null to null', () {
+      expect(type.mapToSqlConstant(null), 'NULL');
+      expect(type.mapToSqlVariable(null), null);
+    });
+  });
+}