Avoid macOS debug messages about invalid glyph index

Don't use NSLayoutManager lineFragmentRectForGlyphAtIndex:effectiveRange
when NSTextView is empty, as this result in debug error messages.

Fall back to using the font height in this case instead, as this seems
to be the only thing to do in this case.

Closes #18793.

src/osx/cocoa/textctrl.mm

@@ -1235,9 +1235,20 @@ wxSize wxNSTextViewControl::GetBestSize() const
         // any non-empty control) and then multiply it by the number of lines
         // we want to have by default, which is currently just hardcoded as 5
         // for compatibility with the behaviour of the previous versions.
-        NSRect rect = [layoutManager lineFragmentRectForGlyphAtIndex: 0
+        CGFloat height;
-                                     effectiveRange: nil];
+        if ( [[m_textView string] length] )
-        size.y = (int)(5*rect.size.height + [m_textView textContainerInset].height);
+        {
+            NSRect rect = [layoutManager lineFragmentRectForGlyphAtIndex: 0
+                                         effectiveRange: nil];
+            height = rect.size.height;
+        }
+        else // The control is empty.
+        {
+            // Not really clear what else could we use here.
+            height = GetWXPeer()->GetCharHeight();
+        }
+
+        size.y = (int)(5*height + [m_textView textContainerInset].height);
     }
 
     return size;