Some plugin might unset template variable `PAGE_BANNER`, resulting in a warning:

```
Undefined array key "PAGE_BANNER"
```
[PHP 8.2] Undefined array key "PAGE_BANNER"
This is a GitHub Issue
repo:Piwigo
owner:Piwigo
Title : [PHP 8.2] Undefined array key "PAGE_BANNER"
Issue date:

--- start body ---
Some plugin might unset template variable `PAGE_BANNER`, resulting in a warning:

```
Undefined array key "PAGE_BANNER"
```
--- end body ---
