fix NPE when null present in output Seq

Author: chrisalbright

src/main/scala/com/github/tototoshi/csv/CSVWriter.scala

@@ -59,7 +59,7 @@ class CSVWriter (protected val writer: Writer)(implicit val format: CSVFormat) {
     def escapeDelimiterChar(field: String): String =
       field.replace(format.delimiter.toString, format.escapeChar.toString + format.delimiter.toString)
 
-    def show(s: Any): String = s.toString
+      def show(s: Any): String = Option(s).getOrElse("").toString
 
     val renderField = {
       val escape = format.quoting match {

src/test/scala/com/github/tototoshi/csv/CSVWriterSpec.scala

@@ -79,6 +79,19 @@ class CSVWriterSpec extends FunSpec with ShouldMatchers with BeforeAndAfter with
 
         readFileAsString("test.csv") should be (expected)
       }
+
+      it("writes null fields as empty strings") {
+        using (CSVWriter.open(new FileWriter("test.csv"))) { writer =>
+          writer.writeAll(List(List("a", null, "c"), List("d", "e", null)))
+        }
+
+        val expected = """|a,,c
+        |d,e,
+        |""".stripMargin
+
+        readFileAsString("test.csv") should be (expected)
+      }
+
       describe ("When stream is already closed") {
         it ("throws an Exception") {
           val writer = CSVWriter.open("test.csv")
@@ -103,6 +116,18 @@ class CSVWriterSpec extends FunSpec with ShouldMatchers with BeforeAndAfter with
 
         readFileAsString("test.csv") should be (expected)
       }
+      it("write single line with null fieldsto file") {
+        using (CSVWriter.open(new FileWriter("test.csv"))) { writer =>
+          writer.writeRow(List("a", null, "c"))
+          writer.writeRow(List("d", "e", null))
+        }
+
+        val expected = """|a,,c
+                         |d,e,
+                         |""".stripMargin
+
+        readFileAsString("test.csv") should be (expected)
+      }
       describe ("When a field contains quoteChar in it") {
         it ("should escape the quoteChar") {
           using (CSVWriter.open(new FileWriter("test.csv"))) { writer =>