I saw the problem like this with another kata a couple days ago. https://www.codewars.com/kata/do-i-get-a-bonus It also contains a Unicode char in tests.
Perhaps it needs to complain to developers of codewars?

My solution passed final test but preliminary tests was failed with exception about assertion error.

It is a dirty hack because objects are not really equal and nobody can use it this way because of possible bugs that very hard to find.

Tests throws assertion error.

It takes me 5 minutes to solve. There is so complex description that solution must be very simple. First try and kata has solved.

Even if I return "" in Java it outputs:

Error: Command failed: javac -verbose -cp /home/codewarrior -d /home/codewarrior -sourcepath /home/codewarrior -cp /usr/local/groovy/lib/junit-4.12.jar:/usr/local/groovy/lib/hamcrest-core-1.3.jar /home/codewarrior/Kata.java /home/codewarrior/BonusTimeTest.java ./frameworks/java/CwRunListener.java
[parsing started RegularFileObject[/home/codewarrior/Kata.java]]
[parsing completed 32ms]
/home/codewarrior/BonusTimeTest.java:31: error: unmappable character for encoding ASCII
System.out.println("\u00A3 == ??:"+"\u00A3".equals("??"));
^
/home/codewarrior/BonusTimeTest.java:31: error: unmappable character for encoding ASCII
System.out.println("\u00A3 == ??:"+"\u00A3".equals("??"));
^
/home/codewarrior/BonusTimeTest.java:31: error: unmappable character for encoding ASCII
System.out.println("\u00A3 == ??:"+"\u00A3".equals("??"));
^
/home/codewarrior/BonusTimeTest.java:31: error: unmappable character for encoding ASCII
System.out.println("\u00A3 == ??:"+"\u00A3".equals("??"));
^
[parsing started RegularFileObject[/home/codewarrior/BonusTimeTest.java]]
[parsing completed 10ms]
[parsing started RegularFileObject[./frameworks/java/CwRunListener.java]]
[parsing completed 1ms]
[total 111ms]
4 errors

Why do you sort in ascending order?