### Steps to reproduce

1. Go to /admin/costs

2. For the currency format, remove %n, enter, for instance, only `€`


<img class="op-uc-image op-uc-image_inline" src="/api/v3/attachments/436465/content">

### What is the buggy behavior?

* it&#39;s possible to not include %n - which is the actual value - in the format string

* this also breaks the frontend which then fails to render any work packages at all


```text
Uncaught TypeError: Cannot read properties of null (reading '0')
at CurrencyDisplayField.isEmpty (currency-display-field.module.ts:35:42)
at DisplayFieldRenderer.getText (display-field-renderer.ts:115:15)
at DisplayFieldRenderer.renderFieldValue (display-field-renderer.ts:79:29)
at DisplayFieldRenderer.render (display-field-renderer.ts:50:32)
at CellBuilder.render (cell-builder.ts:69:47)
at CellBuilder.build (cell-builder.ts:53:10)
at SingleHierarchyRowBuilder.buildCell (single-row-builder.ts:122:33)
at single-row-builder.ts:237:21
 at Array.forEach (<anonymous>)
at SingleHierarchyRowBuilder.buildEmptyRow (single-row-builder.ts:229:27)
```

### What is the expected behavior?

Begin of the insertion* Add field validation so at leastEnd of the insertion Begin of the deletion 1. it should not be allowed to not include `%n` inEnd of the deletion the Begin of the insertion&nbsp;`%n` must be presentEnd of the insertion Begin of the deletion format stringEnd of the deletion

Begin of the insertion* The FormatEnd of the insertion Begin of the deletion 2. theEnd of the deletion field Begin of the insertionhas a link to documentation that helps users understand the use ofEnd of the insertion Begin of the deletion labels should also mention and explain bothEnd of the deletion `%n` and `%u` Begin of the insertion

* &quot;Eg. n%u%, where %n represents the number and %u the unit.&quot;


End of the insertion Begin of the deletion


End of the deletion ### Environment information

**OpenProject version**

_v15.5.0_