fix(sql): strings should be nullable without casting as JSON

packages/mysql/tests/mysql.spec.ts

@@ -323,3 +323,22 @@ test('unique constraint 1', async () => {
         await expect(p).rejects.toBeInstanceOf(UniqueConstraintFailure);
     }
 });
+
+test('string/null unions should not render as JSON', async () => {
+    @entity.name('model6')
+    class Model {
+        id: number & PrimaryKey & AutoIncrement = 0;
+
+        constructor(public name: string, public nickName: string | null = null) {}
+    }
+
+    const database = await databaseFactory([Model]);
+    await database.persist(new Model('Peter'));
+    await database.persist(new Model('Christopher', 'Chris'));
+
+    const result = await database.query(Model).orderBy('id', 'asc').find();
+    expect(result).toMatchObject([
+        {name: 'Peter', nickName: null},
+        {name: 'Christopher', nickName: 'Chris'}
+    ]);
+});

packages/sql/src/platform/default-platform.ts

@@ -31,7 +31,7 @@ export function isNonUndefined(type: Type): boolean {
 export function typeResolvesToString(type: Type): boolean {
     if (type.kind === ReflectionKind.string) return true;
     if (type.kind === ReflectionKind.literal && 'string' === typeof type.literal) return true;
-    if (type.kind === ReflectionKind.union) return type.types.every(v => typeResolvesToString(v));
+    if (type.kind === ReflectionKind.union) return type.types.filter(isNonUndefined).every(v => typeResolvesToString(v));
     if (type.kind === ReflectionKind.enum) return typeResolvesToString(type.indexType);
     return false;
 }