Some tweaks to @Initializer annotations in wala.util (#1474)

- Remove a couple of `@Nullable` annotations and add `@Initializer`
instead, to better capture the protocol.
- Remove WALA's custom `@Initializer` annotation and use the one from
NullAway's annotations package, which we already depend on.

util/src/main/java/com/ibm/wala/util/graph/traverse/DFSDiscoverTimeIterator.java

@@ -15,6 +15,7 @@
 import com.ibm.wala.util.collections.NonNullSingletonIterator;
 import com.ibm.wala.util.debug.UnimplementedError;
 import com.ibm.wala.util.graph.NumberedGraph;
+import com.uber.nullaway.annotations.Initializer;
 import java.util.ArrayList;
 import java.util.Iterator;
 import java.util.NoSuchElementException;
@@ -32,9 +33,10 @@ public abstract class DFSDiscoverTimeIterator<T> extends ArrayList<T> implements
   private static final long serialVersionUID = 4238700455408861924L;
 
   /** an enumeration of all nodes to search from */
-  private @Nullable Iterator<? extends T> roots;
+  private Iterator<? extends T> roots;
 
   /** subclass constructors must call this! */
+  @Initializer
   protected void init(Iterator<? extends T> nodes) {
     roots = nodes;
     assert nodes != null;

util/src/main/java/com/ibm/wala/util/graph/traverse/DFSFinishTimeIterator.java

@@ -10,11 +10,11 @@
  */
 package com.ibm.wala.util.graph.traverse;
 
-import com.ibm.wala.qual.Initializer;
 import com.ibm.wala.util.collections.EmptyIterator;
 import com.ibm.wala.util.collections.Iterator2Iterable;
 import com.ibm.wala.util.debug.UnimplementedError;
 import com.ibm.wala.util.graph.Graph;
+import com.uber.nullaway.annotations.Initializer;
 import java.util.ArrayList;
 import java.util.Iterator;
 import java.util.NoSuchElementException;

util/src/main/java/com/ibm/wala/util/graph/traverse/GraphDFSDiscoverTimeIterator.java

@@ -11,17 +11,17 @@
 package com.ibm.wala.util.graph.traverse;
 
 import com.ibm.wala.util.graph.Graph;
+import com.uber.nullaway.annotations.Initializer;
 import java.util.Iterator;
-import org.jspecify.annotations.NullUnmarked;
-import org.jspecify.annotations.Nullable;
 
 abstract class GraphDFSDiscoverTimeIterator<T> extends DFSDiscoverTimeIterator<T> {
 
   private static final long serialVersionUID = -5673397879499010863L;
 
   /** the graph being searched */
-  private @Nullable Graph<T> G;
+  private Graph<T> G;
 
+  @Initializer
   protected void init(Graph<T> G, Iterator<? extends T> nodes) {
     if (G == null) {
       throw new IllegalArgumentException("G is null");
@@ -30,7 +30,6 @@ protected void init(Graph<T> G, Iterator<? extends T> nodes) {
     super.init(nodes);
   }
 
-  @NullUnmarked
   @Override
   protected Iterator<? extends T> getConnected(T n) {
     return G.getSuccNodes(n);