Fix blank topic on explore repo list (#3956)

templates/explore/repo_list.tmpl

@@ -17,9 +17,13 @@
 				</div>
 			</div>
 			{{if .DescriptionHTML}}<p class="has-emoji">{{.DescriptionHTML}}</p>{{end}}
-			<div>
-			{{range .Topics}}<div class="ui green basic label topic">{{.}}</div>{{end}}
-			</div>
+			{{if .Topics }}
+				<div>
+				{{range .Topics}}
+					{{if ne . "" }}<div class="ui green basic label topic">{{.}}</div>{{end}}
+				{{end}}
+				</div>
+			{{end}}
 			<p class="time">{{$.i18n.Tr "org.repo_updated"}} {{TimeSinceUnix .UpdatedUnix $.i18n.Lang}}</p>
 		</div>
 	{{else}}