Prevent adding nil label to .AddedLabels or .RemovedLabels (#14623)

* Prevent adding nil label to .AddedLabels or .RemovedLabels

There are possibly a few old databases out there with malmigrated data that can
cause panics with empty labels being migrated.

This PR adds a few tests to prevent nil labels being added.

Fix #14466

Signed-off-by: Andrew Thornton <art27@cantab.net>

* Add doctor command to remove the broken label comments

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: 6543 <6543@obermui.de>

modules/templates/helper.go

@@ -371,6 +371,10 @@ func NewFuncMap() []template.FuncMap {
 		"RenderLabels": func(labels []*models.Label) template.HTML {
 			html := `<span class="labels-list">`
 			for _, label := range labels {
+				// Protect against nil value in labels - shouldn't happen but would cause a panic if so
+				if label == nil {
+					continue
+				}
 				html += fmt.Sprintf("<div class='ui label' style='color: %s; background-color: %s'>%s</div> ",
 					label.ForegroundColor(), label.Color, RenderEmoji(label.Name))
 			}