Add Preconditions check to help assess a NPE we saw.

The NPE seems impossible, but let's see if this Preconditions check fires and shows us it's actually possible.

PiperOrigin-RevId: 642260180
Change-Id: I0a6ae3c96ff56798b5f108b14229afb09a9c84d1

src/main/java/com/google/devtools/build/skyframe/KeyToConsolidate.java

@@ -106,6 +106,7 @@ static SkyKey key(Object obj) {
    * to {@link #op} when decoding an operation emitted by this method.
    */
   static Object create(SkyKey key, Op op, IncrementalInMemoryNodeEntry entry) {
+    Preconditions.checkNotNull(key);
     if (op == ReverseDepsUtility.getOpToStoreBare(entry)) {
       return key;
     }