Polish "Fix collectionToDelimitedString failure for null elements."

Fixes gh-27419

spring-core/src/test/java/org/springframework/util/StringUtilsTests.java

@@ -17,6 +17,7 @@
 package org.springframework.util;
 
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.Locale;
 import java.util.Properties;
 
@@ -777,4 +778,9 @@ class StringUtilsTests {
 		assertThat(StringUtils.split(null, null)).isNull();
 	}
 
+	@Test
+	void collectionToDelimitedStringWithNullValuesShouldNotFail() {
+		assertThat(StringUtils.collectionToCommaDelimitedString(Collections.singletonList(null))).isEqualTo("null");
+	}
+
 }