Homework: Listings should not require an image

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- As a user creating a listing, I should not be required to upload an image.
- As a user viewing a listing, if no listing image is provided, a default image should be displayed

NOTES:
- There is a `no-photo.jpg` in the `assets` folder that can be used
```

```
- The API call to create a listing without an image is:
```
POST /api/listings
BODY
{
"name": "This is the post",
"description": "The is a description of the post. It contains details about it.",
"price": 42,
"tags": [
"tag1",
"tag2",
"tag3",
"tag4",
"tag5"
]
}
```

Start by locating the listing creation form and the listing display entry point, then trace the POST /api/listings request to see how image data is currently handled. Use assets/no-photo.jpg when a listing has no image, allow creation without an upload, and verify both creation and display behavior with the available project checks.

Written by the indexing model from the issue text.