Avoid unnecessary calls to Collections.emptySet

IntelliJ IDEA rewrote these Map manipulations to use Map.getOrDefault in
ways that always called Collections.emptySet even when no such empty set
was required.  I've manually replaced those with rewrites that avoid
unnecessary calls to Collections.emptySet in exchange for assuming that
the Maps never contain null values.

com.ibm.wala.cast/source/java/com/ibm/wala/cast/tree/impl/CAstControlFlowRecorder.java

@@ -100,12 +100,14 @@ public CAstNode getTarget(CAstNode from, Object label) {
   @Override
   public Collection<Object> getTargetLabels(CAstNode from) {
     Object node = CAstToNode.get(from);
-    return labelMap.getOrDefault(node, Collections.emptySet());
+    Set<Object> found = labelMap.get(node);
+    return found == null ? Collections.emptySet() : found;
   }
 
   @Override
   public Set<Object> getSourceNodes(CAstNode to) {
-    return sourceMap.getOrDefault(CAstToNode.get(to), Collections.emptySet());
+    Set<Object> found = sourceMap.get(CAstToNode.get(to));
+    return found == null ? Collections.emptySet() : found;
   }
 
   @Override