Prevent NPE when viewing non-rendered files (#18234)

Fix #18231

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

templates/repo/unicode_escape_prompt.tmpl

@@ -1,3 +1,4 @@
+{{if .EscapeStatus}}
 	{{if .EscapeStatus.BadBIDI}}
 		<div class="ui error message unicode-escape-prompt">
 			<span class="close icon hide-panel button" data-panel-closest=".message">{{svg "octicon-x" 16 "close inside"}}</span>
@@ -15,3 +16,4 @@
 			<p>{{$.root.i18n.Tr "repo.unicode_description" | Str2html}}</p>
 		</div>
 	{{end}}
+{{end}}