Fix NPE in RemoteConfiguredTargetValue after discarding cache.

Override `isCleared()` in `RemoteConfiguredTargetValue` to correctly report
when the remote configured target has been cleared. This allows
`SkyframeExecutor.dropConfiguredTargetsNow` to drop these cleared nodes
at the start of a build rather than reusing them as `DONE` nodes with a
null `configuredTarget`, which leads to NullPointerException.

PiperOrigin-RevId: 934090115
Change-Id: I5b249b247fee445508b256d2cf4f5be283eba15a

Author: aoeui

src/main/java/com/google/devtools/build/lib/skyframe/RemoteConfiguredTargetValue.java

@@ -88,6 +88,11 @@ public final TargetData getTargetData() {
     return targetData;
   }
 
+  @Override
+  public final boolean isCleared() {
+    return configuredTarget == null;
+  }
+
   @Override
   public final String toString() {
     return toStringHelper(this)