fixup! Implement model for String.equals

Author: romainbrenguier

src/main/java/java/lang/String.java

@@ -1633,7 +1633,7 @@ public int compareTo(String anotherString) {
     // DIFFBLUE MODEL LIBRARY For some reason this needs to be not null for
     // FileReader tests to pass.
     public static final Comparator<String> CASE_INSENSITIVE_ORDER
-                                            = null;
+                                            = CProver.nondetWithoutNullForNotModelled();
     // DIFFBLUE MODEL LIBRARY Not needed for modelling
     // private static class CaseInsensitiveComparator
     //         implements Comparator<String>, java.io.Serializable {