Polish 62a2e65

Add null check for delimiter

Author: fmbenhassine

spring-batch-infrastructure/src/main/java/org/springframework/batch/item/file/builder/FlatFileItemWriterBuilder.java

@@ -463,7 +463,9 @@ public DelimitedLineAggregator<T> build() {
 					"A list of field names or a field extractor is required");
 
 			DelimitedLineAggregator<T> delimitedLineAggregator = new DelimitedLineAggregator<>();
-			delimitedLineAggregator.setDelimiter(this.delimiter);
+			if (this.delimiter != null) {
+				delimitedLineAggregator.setDelimiter(this.delimiter);
+			}
 
 			if (this.fieldExtractor == null) {
 				BeanWrapperFieldExtractor<T> beanWrapperFieldExtractor = new BeanWrapperFieldExtractor<>();