Ajax call gives error with empty images (#1075)

As a result of commit 8d69a59 made to solve Issue 207440, the case of handling empty images in ajax calls raised in Issue 207532 was broken.

A change is made to the original change so that the two cases raised work.

Issue: 207532

Author: iroqueta

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

@@ -854,11 +854,7 @@ public static boolean dateCompare( Date left , Date right)
    	public static int strcmp( String left ,
                              String right )
    	{
-		return Comparator.nullsFirst(String::compareTo)
-			.compare(
-				left  == null ? null : rtrim(left),
-				right == null ? null : rtrim(right)
-			);
+		return rtrim(left).compareTo(rtrim(right));
 	}
 
    	public static boolean strcmp2( String left ,

java/src/main/java/com/genexus/internet/HttpContext.java

@@ -727,7 +727,7 @@ public String getBuildNumber(int buildN){
 	public String convertURL(String file)
 	{
 		String url = "";
-		if (file.equals(""))
+		if (file == null || file.equals(""))
 		{
 			return "";
 		}