Fix auto-sizing multiline wxGrid column labels with empty lines

Account for the empty lines explicitly by reserving enough vertical
space for them, as wxDC::GetTextExtent() wouldn't do it as it simply
returns 0 for empty strings.

Closes #18028.

src/generic/grid.cpp

@@ -6138,11 +6138,20 @@ void wxGrid::GetTextBoxSize( const wxDC& dc,
 
     size_t i;
     for ( i = 0; i < lines.GetCount(); i++ )
+    {
+        if ( lines[i].empty() )
+        {
+            // GetTextExtent() would return 0 for empty lines, but we still
+            // need to account for their height.
+            h += dc.GetCharHeight();
+        }
+        else
         {
             dc.GetTextExtent( lines[i], &lineW, &lineH );
             w = wxMax( w, lineW );
             h += lineH;
         }
+    }
 
     *width = w;
     *height = h;