Postgresql: Updating an existing field with adding/incrementing some value didn't work as expected
I found that I'm concatenating a string with a None type object...
```python
# Work.authors = None
# author = 'string'
Work.update(authors=Work.authors + author)
```
Maybe this is the problem I'm encountering? It's strange that I'm not getting any errors though All is working as far as I can tell:

```python
db = PostgresqlDatabase('peewee_test')

class Reg(Model):
    data = TextField()
    class Meta:
        database = db

db.drop_tables([Reg])
db.create_tables([Reg])

r = Reg.create(data='start')
uq = (Reg
      .update(data=Reg.data + '-end')
      .where(Reg.id == r.id)
      .execute())

r_db = Reg.get(Reg.id == r.id)
print(r_db.data)
```

Prints:

```
start-end
``` Oh, sorry for troubling
Maybe something went wrong, and it seems I haven't tested my code thoroughly yet
Thank you for your confirmation on this case, I really love this library!
My table is:
```python
class Work(BaseModel):
    id = AutoField()
    user_id = BigIntegerField()
    name = TextField(unique=True)
    filename = CharField(null=True, unique=True)
    authors = TextField(null=True)
    content = TextField(null=True)
    start_date = DateField()
    end_date = DateField(null=True)
```

I'm trying to add (that is, +=) some string to an existing data record:
```python
def add_author(user_id: int, name: str, author: str):
    # author = 'Illyustrator:An illustrator%'
    # the delimiter is '%'
    return Work.update(authors=Work.authors + author).where(
            (Work.user_id == user_id) & (Work.name == name)
    ).execute()
```

The debug logs are:
```
2022-10-01 22:41:17,493 - DEBUG - peewee - ('SELECT "t1"."authors" FROM "work" AS "t1" WHERE (("t1"."user_id" = %s) AND ("t1"."name" = %s))', [5581756786, 'Title name here'])
2022-10-01 22:41:17,495 - DEBUG - peewee - ('UPDATE "work" SET "authors" = ("work"."authors" || %s) WHERE (("work"."user_id" = %s) AND ("work"."name" = %s))', ['Illyustrator:An illustrator%', 5581756786, 'Title name here'])
```

a Postgresql query in the psql cli after that query:
```
ebookmaker=> select * from work;
 id |  user_id   |      name       | filename | authors | content | start_date | end_date 
----+------------+-----------------+----------+---------+---------+------------+----------
  4 | 5581756786 | Title name here |          |         |         | 2022-10-01 | 
(1 row)
```
But the query doesn't adding the string value to the authors field...
Versions of python and pewee are:
```
Python 3.10.6 (main, Aug  3 2022, 17:39:45) [GCC 12.1.1 20220730] on linux
peewee: 3.15.2
```

By the way, this feature was working in sqlite database