fix: use celldata cache for merged cell styles (#8330) (#8378)

Co-authored-by: Ugur Saglam <106508695+ugur-vaadin@users.noreply.github.com>

Author: vaadin-bot

vaadin-spreadsheet-flow-parent/vaadin-spreadsheet-flow-client/src/main/java/com/vaadin/addon/spreadsheet/client/SheetWidget.java

@@ -3645,11 +3645,7 @@ public void addMergedRegion(MergedRegion region) {
         }
         String key = toKey(region.col1, region.row1);
         MergedCell mergedCell = new MergedCell(this, region.col1, region.row1);
-        String cellStyle = "cs0";
-        Cell cell = getCell(region.col1, region.row1);
-        if (cell != null) {
-            cellStyle = cell.getCellStyle();
-        }
+        String cellStyle = getMergedRegionCellStyle(region);
         mergedCell.setValue(getCellValue(region.col1, region.row1), cellStyle,
                 false);
         DivElement element = mergedCell.getElement();
@@ -3678,6 +3674,18 @@ public void addMergedRegion(MergedRegion region) {
         }
     }
 
+    private String getMergedRegionCellStyle(MergedRegion region) {
+        CellData cellData = getCellData(region.col1, region.row1);
+        if (cellData != null && cellData.cellStyle != null) {
+            return cellData.cellStyle;
+        }
+        Cell cell = getCell(region.col1, region.row1);
+        if (cell != null) {
+            return cell.getCellStyle();
+        }
+        return "cs0";
+    }
+
     /**
      * For internal use only! May be removed in the future.
      */