fix checkerframework

Author: wadoon

key.util/src/main/java/org/key_project/util/collection/ImmutableList.java

@@ -126,10 +126,12 @@ public interface ImmutableList<T extends @Nullable Object>
         return seq.stream().map(Objects::toString).collect(Collectors.joining(", ", "[", "]"));
     }
 
-    static <T extends @Nullable Object> int hash(ImmutableList<T> seq) {
+    static <T extends @Nullable Object> int hash(@Nullable ImmutableList<T> seq) {
         int hash = 0;
-        for (var e : seq) {
-            hash = 31 * hash + e.hashCode();
+        if (seq != null) {
+            for (var e : seq) {
+                hash = 31 * hash + (e == null ? 0 : e.hashCode());
+            }
         }
         return hash;
     }