I think it's better to fix it in template, but not add empty values to the map. Because if there is no details, the template shouldn't render that part.

(And in case the future developers would forget/not know that there must be a detail to make the template work ....)

{{.Message}}
<details>
	<summary>{{.Summary}}</summary>
	{{if .Details}}<code>{{.Details | SanitizeHTML}}</code>{{end}}
</details>

This works but there's a downside of there still being the drop down arrow for it without any contents:

I came up with this to fix it:

{{.Message}}
{{ if .Details }}
<details>
	<summary>{{.Summary}}</summary>
	<code>{{.Details | SanitizeHTML}}</code>
</details>
{{ else }}
<div>
	{{ .Summary }}
</div>
{{ end }}

Let me know if that works or if I should leave the version with empty contents. I'm not sure if that has any impact somewhere else.

I came up with this to fix it:

Yup, that's even better. Thank you!

I think this could be backport to v1.23