Don't evaluate filterKnownInputs() twice for no reason.

Possibly we could also check if the return value is empty and just set
state.discoveredInputs to an empty list, but I think a skyframe restart
after this point is unlikely anyway.

RELNOTES: None.
PiperOrigin-RevId: 210170567

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

@@ -524,7 +524,7 @@ private ActionExecutionValue checkCacheAndExecuteIfNeeded(
       if (env.valuesMissing()) {
         return null;
       }
-      if (!Iterables.isEmpty(newInputs)) {
+      if (!metadataFoundDuringActionExecution.isEmpty()) {
         // We are in the interesting case of an action that discovered its inputs during
         // execution, and found some new ones, but the new ones were already present in the graph.
         // We must therefore cache the metadata for those new ones.