Fix: SheetTransformer

Author: psainics

src/main/java/io/cdap/plugin/google/sheets/source/SheetTransformer.java

@@ -69,7 +69,8 @@ public static StructuredRecord transform(RowRecord rowRecord, Schema schema, boo
         builder.set(metadataRecordName, rowRecord.getMetadata());
       } else {
         ComplexSingleValueColumn complexSingleValueColumn = rowRecord.getHeaderedCells().get(name);
-        if (complexSingleValueColumn == null || complexSingleValueColumn.getData() == null) {
+        if (complexSingleValueColumn == null || complexSingleValueColumn.getData() == null
+            || complexSingleValueColumn.getSubColumns().isEmpty()) {
           builder.set(name, null);
         } else {
           processCellData(builder, field, complexSingleValueColumn);