Allow nullable Java Time parameters

Issue: junit-team#4010

junit-jupiter-params/src/main/java/org/junit/jupiter/params/converter/JavaTimeArgumentConverter.java

@@ -52,7 +52,11 @@ class JavaTimeArgumentConverter extends AnnotationBasedArgumentConverter<JavaTim
 	@Override
 	protected Object convert(Object input, Class<?> targetClass, JavaTimeConversionPattern annotation) {
 		if (input == null) {
-			throw new ArgumentConversionException("Cannot convert null to " + targetClass.getName());
+			if (annotation.nullable()) {
+				return null;
+			}
+			throw new ArgumentConversionException(
+				"Cannot convert null to " + targetClass.getName() + ", consider setting 'nullable = true'");
 		}
 		TemporalQuery<?> temporalQuery = TEMPORAL_QUERIES.get(targetClass);
 		if (temporalQuery == null) {

junit-jupiter-params/src/main/java/org/junit/jupiter/params/converter/JavaTimeConversionPattern.java

@@ -45,4 +45,9 @@
 	 */
 	String value();
 
+	/**
+	 * Whether this Java Time parameter may be null or not.
+	 */
+	boolean nullable() default false;
+
 }

jupiter-tests/src/test/java/org/junit/jupiter/params/converter/JavaTimeArgumentConverterTests.java

@@ -110,10 +110,29 @@ void throwsExceptionOnInvalidTargetType() {
 		assertThat(exception).hasMessage("Cannot convert to java.lang.Integer: 2017");
 	}
 
+	@Test
+	void throwsExceptionOnNullParameterWithoutNullable() {
+		var exception = assertThrows(ArgumentConversionException.class,
+			() -> convert(null, "dd.MM.yyyy", LocalDate.class));
+
+		assertThat(exception).hasMessage(
+			"Cannot convert null to java.time.LocalDate, consider setting 'nullable = true'");
+	}
+
+	@Test
+	void convertsNullableParameter() {
+		assertThat(convert(null, "dd.MM.yyyy", true, LocalDate.class)).isEqualTo(null);
+	}
+
 	private Object convert(Object input, String pattern, Class<?> targetClass) {
+		return convert(input, pattern, false, targetClass);
+	}
+
+	private Object convert(Object input, String pattern, boolean nullable, Class<?> targetClass) {
 		var converter = new JavaTimeArgumentConverter();
 		var annotation = mock(JavaTimeConversionPattern.class);
 		when(annotation.value()).thenReturn(pattern);
+		when(annotation.nullable()).thenReturn(nullable);
 
 		return converter.convert(input, targetClass, annotation);
 	}