Support nullable flag value in ShortcutConfigurable.ShortcutType#GATHER_LIST_TAIL_FLAG

Fixes gh-3049

Author: muchnik

spring-cloud-gateway-server/src/main/java/org/springframework/cloud/gateway/support/ShortcutConfigurable.java

@@ -151,7 +151,7 @@ public Map<String, Object> normalize(Map<String, String> args, ShortcutConfigura
 					// strip boolean flag if last entry is true or false
 					int lastIdx = values.size() - 1;
 					String lastValue = values.get(lastIdx);
-					if (lastValue.equalsIgnoreCase("true") || lastValue.equalsIgnoreCase("false")) {
+					if ("true".equalsIgnoreCase(lastValue) || "false".equalsIgnoreCase(lastValue) || lastValue == null) {
 						values = values.subList(0, lastIdx);
 						map.put(fieldOrder.get(1), getValue(parser, beanFactory, lastValue));
 					}

spring-cloud-gateway-server/src/test/java/org/springframework/cloud/gateway/support/ShortcutConfigurableTests.java

@@ -182,6 +182,32 @@ public ShortcutType shortcutType() {
 		}
 	}
 
+	@Test
+	public void testNormalizeGatherListTailFlagFlagIsNull() {
+		parser = new SpelExpressionParser();
+		ShortcutConfigurable shortcutConfigurable = new ShortcutConfigurable() {
+			@Override
+			public List<String> shortcutFieldOrder() {
+				return Arrays.asList("values", "flag");
+			}
+
+			@Override
+			public ShortcutType shortcutType() {
+				return ShortcutType.GATHER_LIST_TAIL_FLAG;
+			}
+		};
+		Map<String, String> args = new HashMap<>();
+		args.put("1", "val0");
+		args.put("2", "val1");
+		args.put("3", "val2");
+		args.put("4", null);
+		Map<String, Object> map = ShortcutType.GATHER_LIST_TAIL_FLAG.normalize(args, shortcutConfigurable, parser,
+				this.beanFactory);
+		assertThat(map).isNotNull().containsKey("values");
+		assertThat((List) map.get("values")).containsExactly("val0", "val1", "val2");
+		assertThat(map.get("flag")).isNull();
+	}
+
 	@SpringBootConfiguration
 	protected static class TestConfig {