Add test for CSV parser using null as an empty value parser result

This commit describes and tests the status-quo of the current
implementation that return `null` for empty CSV values. It might be
changed in the future.

Addresses #1014

Author: sormuras

junit-jupiter-params/src/main/java/org/junit/jupiter/params/provider/CsvArgumentsProvider.java

@@ -41,6 +41,7 @@ public Stream<? extends Arguments> provideArguments(ExtensionContext context) {
 		settings.getFormat().setDelimiter(delimiter);
 		settings.getFormat().setQuote('\'');
 		settings.getFormat().setQuoteEscape('\'');
+		settings.setEmptyValue(null); // see https://github.com/junit-team/junit5/issues/1014
 		settings.setAutoConfigurationEnabled(false);
 		CsvParser csvParser = new CsvParser(settings);
 		AtomicLong index = new AtomicLong(0);

junit-jupiter-params/src/test/java/org/junit/jupiter/params/provider/CsvArgumentsProviderTests.java

@@ -68,6 +68,13 @@ void throwsExceptionOnInvalidCsv() {
 		assertThat(exception).hasMessage("Line at index 2 contains invalid CSV: \"\"");
 	}
 
+	@Test
+	void emptyValueIsNull() {
+		Stream<Object[]> arguments = provideArguments(',', "null , , empty , ''");
+
+		assertThat(arguments).containsExactly(new String[] { "null", null, "empty", null });
+	}
+
 	private Stream<Object[]> provideArguments(char delimiter, String... value) {
 		CsvSource annotation = mock(CsvSource.class);
 		when(annotation.value()).thenReturn(value);