pref(table): prevent unnecessary reflow with sticky row (#12786)

The offset of a sticky row is calculated by summing up the heights of
the previous sticky rows. As the access to the height causes a forced
reflow this is now skipped when there is only one row.

Author: nename0

src/cdk/table/sticky-styler.ts

@@ -140,6 +140,10 @@ export class StickyStyler {
         this._addStickyStyle(row, position, stickyHeight);
       }
 
+      if (rowIndex === rows.length - 1) {
+        // prevent unnecessary reflow from getBoundingClientRect()
+        return;
+      }
       stickyHeight += row.getBoundingClientRect().height;
     }
   }