Fix checkerframework issues

Author: wadoon

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

@@ -162,7 +162,7 @@ public Iterator<S> iterator() {
         return new ArrayIterator<>(this);
     }
 
-    private static class ArrayIterator<T> implements Iterator<T> {
+    private static class ArrayIterator<T extends @Nullable Object> implements Iterator<T> {
 
         private int i = 0;
         private final ImmutableArray<T> coll;

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

@@ -163,7 +163,7 @@ private Immutables() {
      *
      * @return the view onto the iterable as an immutable list
      */
-    public static <T> ImmutableList<T> createListFrom(Iterable<? extends T> iterable) {
+    public static <T extends @Nullable Object> ImmutableList<T> createListFrom(Iterable<? extends T> iterable) {
         ImmutableList<T> result = ImmutableSLList.nil();
         for (T t : iterable) {
             result = result.prepend(t);

settings.gradle

@@ -19,4 +19,4 @@ include 'key.format'
 
 // ENABLE NULLNESS here or on the CLI
 // This flag is activated to enable the checker framework.
-// System.setProperty("ENABLE_NULLNESS", "true")
+System.setProperty("ENABLE_NULLNESS", "true")