Fix some RenderShortcodes error cases

This issue fixes two cases where `{{__hugo_ctx` artifacts were left in the rendered output:

1. Inclusion when `.RenderShortcodes` is wrapped in HTML.
2. Inclusion of Markdown file without a trailing newline in some cases.

Closes #12854
Updates #12998

markup/goldmark/hugocontext/hugocontext_test.go

@@ -24,7 +24,7 @@ func TestWrap(t *testing.T) {
 
 	b := []byte("test")
 
-	c.Assert(Wrap(b, 42), qt.Equals, "{{__hugo_ctx pid=42}}\ntest{{__hugo_ctx/}}\n")
+	c.Assert(Wrap(b, 42), qt.Equals, "{{__hugo_ctx pid=42}}\ntest\n{{__hugo_ctx/}}\n")
 }
 
 func BenchmarkWrap(b *testing.B) {