Add dedicated class for empty placeholders (#27788)

Fixes: https://github.com/go-gitea/gitea/issues/27784

<img width="1033" alt="Screenshot 2023-10-25 at 19 07 15"
src="https://github.com/go-gitea/gitea/assets/115237/1a363851-1a86-48cb-99ec-0a573371bb6e">
<img width="1051" alt="Screenshot 2023-10-25 at 19 07 41"
src="https://github.com/go-gitea/gitea/assets/115237/add4b606-2264-430a-af35-249ef005817f">

Co-authored-by: KN4CK3R <admin@oldschoolhack.me>

templates/package/shared/list.tmpl

@@ -38,7 +38,7 @@
 	</div>
 	{{else}}
 		{{if not .HasPackages}}
-			<div class="empty center">
+			<div class="empty-placeholder">
 				{{svg "octicon-package" 48}}
 				<h2>{{ctx.Locale.Tr "packages.empty"}}</h2>
 				{{if and .Repository .CanWritePackages}}

templates/repo/actions/runs_list.tmpl

@@ -1,6 +1,6 @@
 <div class="flex-list">
 	{{if eq (len .Runs) 0}}
-	<div class="empty center">
+	<div class="empty-placeholder">
 		{{svg "octicon-no-entry" 48}}
 		<h2>{{if $.IsFiltered}}{{ctx.Locale.Tr "actions.runs.no_results"}}{{else}}{{ctx.Locale.Tr "actions.runs.no_runs"}}{{end}}</h2>
 	</div>

web_src/css/repo.css

@@ -1798,10 +1798,12 @@
   flex: 1
 }
 
-.repository.packages .empty,
-.repository.actions .empty {
-  padding-top: 70px;
-  padding-bottom: 100px;
+.empty-placeholder {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  padding-top: 40px;
+  padding-bottom: 40px;
 }
 
 .repository.packages .file-size {