Avoid nilpointer when shortcode page content output nil

Updates #10391

hugolib/shortcode.go

@@ -381,6 +381,17 @@ func renderShortcode(
 		// Pre Hugo 0.55 this was the behaviour even for the outer-most
 		// shortcode.
 		if sc.doMarkup && (level > 0 || sc.configVersion() == 1) {
+
+			cp := p.pageOutput.cp
+			if cp == nil {
+				var err error
+				cp, err = newPageContentOutput(p, p.pageOutput)
+				if err != nil {
+					return "", false, err
+				}
+				p.pageOutput.initContentProvider(cp)
+			}
+
 			var err error
 			b, err := p.pageOutput.cp.renderContent([]byte(inner), false)
 			if err != nil {