add null check complexSubColumn == null

Author: psainics

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

@@ -129,7 +129,7 @@ private static StructuredRecord processRecord(Schema fieldSchema, ComplexSingleV
     for (Schema.Field subField : fieldSchema.getFields()) {
       String subFieldName = subField.getName();
       ComplexSingleValueColumn complexSubColumn = complexSingleValueColumn.getSubColumns().get(subFieldName);
-      if (complexSubColumn.getData() == null) {
+      if (complexSubColumn == null || complexSubColumn.getData() == null) {
         builder.set(subFieldName, null);
       } else {
         processCellData(builder, subField, complexSubColumn);