Fix merge button dropdown triangle alignment (#14041)

https://github.com/go-gitea/gitea/pull/14028 had improved dropdown
triangle display for buttons with text but had regressed it for buttons
without. Fix this by adding a class to prevent horizontal margin adjustment.

Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

templates/repo/issue/view_content/pull.tmpl

@@ -343,7 +343,7 @@
 										</span>
 									</button>
 									{{if gt $prUnit.PullRequestsConfig.AllowedMergeStyleCount 1}}
-										<div class="ui dropdown icon button">
+										<div class="ui dropdown icon button no-text">
 											{{svg "octicon-triangle-down" 14 "dropdown icon"}}
 											<div class="menu">
 												{{if $prUnit.PullRequestsConfig.AllowMerge}}

web_src/less/_base.less

@@ -1792,6 +1792,10 @@ table th[data-sortt-desc] {
 .ui.selection.dropdown > .dropdown.icon {
   top: 0 !important;
 }
+.ui.dropdown.no-text > .dropdown.icon {
+  margin-left: 0 !important;
+  margin-right: 0 !important;
+}
 
 /* limit width of all direct dropdown menu children */
 /* https://github.com/go-gitea/gitea/pull/10835 */