fix: drop "No article selected" flash on back from article

ClearArticle nulls state before exit transition ends, so the outgoing screen briefly rendered the null-branch fallback in the title bar. Render empty title/content for null state; remove article_no_selected.

app/src/main/res/values/strings.xml

@@ -13,7 +13,6 @@
     <string name="language_russian">Russian</string>
     <string name="language_system">System</string>
     <string name="clear_search">Clear search</string>
-    <string name="article_no_selected">No article selected</string>
     <string name="article_return_to_search">Return to search</string>
     <string name="error_checking_dictionaries">Failed to check dictionary status</string>
     <string name="error_loading_article">Loading error: %1$s</string>