Fix strcmp treating null and empty strings as equal (#1069)

* Fix strcmp treating null and empty strings as equal

Previously, strcmp could return equality when comparing a null value
with an empty string.

This happened because rtrim(null) returned an empty string, causing
null and "" to be treated as equivalent during comparison.

This change preserves the semantic difference between null and empty
strings by handling null values explicitly and delegating comparison
to Comparator.nullsFirst after trimming.

Behavior:
- null is considered less than any non-null String
- empty strings are not equal to null
- trailing whitespace is ignored during comparison

Issue: 207440

* A json with a string field with value "null" was being serialized as ""

Author: iroqueta

common/src/main/java/com/genexus/CommonUtil.java

@@ -19,6 +19,7 @@
 import java.math.BigInteger;
 import java.net.HttpURLConnection;
 import java.net.URL;
+import java.util.Comparator;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 
@@ -853,8 +854,12 @@ public static boolean dateCompare( Date left , Date right)
    	public static int strcmp( String left ,
                              String right )
    	{
-               return rtrim(left).compareTo(rtrim(right));
-   	}
+		return Comparator.nullsFirst(String::compareTo)
+			.compare(
+				left  == null ? null : rtrim(left),
+				right == null ? null : rtrim(right)
+			);
+	}
 
    	public static boolean strcmp2( String left ,
                              String right )
@@ -2804,7 +2809,7 @@ else if (className.equals("long") || className.equals("java.lang.Long") || class
         }
         else if (className.equals("string") || className.indexOf("java.lang.String") != -1)
         {
-            return objStr.equals("null") ? null : objStr;
+			return (obj == JSONObject.NULL)? null : objStr;
         }
         else if (className.equals("double") || className.equals("java.lang.Double") || className.equals("[D"))
         {

java/src/test/java/com/genexus/TestCommonUtil.java

@@ -295,7 +295,7 @@ public void testConvertObjectTo() {
 		try{
 			Class stringClass = Class.forName("java.lang.String");
 			result = CommonUtil.convertObjectTo(obj, stringClass, true);
-			Assert.assertEquals(null, result);
+			Assert.assertEquals("null", result);
 		} catch (Exception e){
 			Assert.fail("Test failed " + e);
 		}