fix: toCommaDelimitedString returns "null,null" when inputs are null (#15338)

* fix: return 'one' if others is null in toCommaDelimitedString

* fix:

Author: ankitshokeen

dubbo-common/src/main/java/org/apache/dubbo/common/utils/StringUtils.java

@@ -1242,14 +1242,20 @@ public static byte decodeHexByte(CharSequence s, int pos) {
     }
 
     /**
-     * Create the common-delimited {@link String} by one or more {@link String} members
+     * Creates a comma-delimited string from one or more string values.
      *
-     * @param one    one {@link String}
-     * @param others others {@link String}
-     * @return <code>null</code> if <code>one</code> or <code>others</code> is <code>null</code>
+     * @param one    the first string value
+     * @param others additional string values
+     * @return the combined string, or null if the first value is null
      * @since 2.7.8
      */
     public static String toCommaDelimitedString(String one, String... others) {
+        if (one == null) {
+            return null;
+        }
+        if (others == null) {
+            return one;
+        }
         String another = arrayToDelimitedString(others, COMMA_SEPARATOR);
         return isEmpty(another) ? one : one + COMMA_SEPARATOR + another;
     }

dubbo-common/src/test/java/org/apache/dubbo/common/utils/StringUtilsTest.java

@@ -474,6 +474,7 @@ private void assertEqualsWithoutSpaces(String expect, String actual) {
 
     /**
      * Test {@link StringUtils#toCommaDelimitedString(String, String...)}
+     *
      * @since 2.7.8
      */
     @Test
@@ -484,6 +485,9 @@ void testToCommaDelimitedString() {
         value = toCommaDelimitedString(null, null);
         assertNull(value);
 
+        value = toCommaDelimitedString("one", null);
+        assertEquals("one", value);
+
         value = toCommaDelimitedString("");
         assertEquals("", value);