[ios] The secondary message of PopoverLabelViewController doesn't show up

The condition of showing the secondary message was set incorrectly.

Bug: 1119859
Change-Id: Ifddaa442877f5219571b0fac0971a370be4e7303
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2366116
Commit-Queue: Tina Wang <tinazwang@chromium.org>
Reviewed-by: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800200}

ios/chrome/common/ui/elements/popover_label_view_controller.mm

@@ -113,10 +113,9 @@ constexpr CGFloat kVerticalDistance = 24;
 
   [_scrollView addSubview:textView];
 
-  // Only create secondary TextView when |secondaryAttributedString| is not nil.
-  // Set the constraint accordingly.
-  if (self.secondaryAttributedString &&
-      !self.secondaryAttributedString.length) {
+  // Only create secondary TextView when |secondaryAttributedString| is not nil
+  // or empty. Set the constraint accordingly.
+  if (self.secondaryAttributedString.length) {
     UITextView* secondaryTextView = [[UITextView alloc] init];
     secondaryTextView.scrollEnabled = NO;
     secondaryTextView.editable = NO;