Set --experimental_action_listeners to default in exec config

Previously, the flag value was set to `null` rather than an empty list, which resulted in distinct yet equivalent configurations.

Fixes bazelbuild#16911

Closes bazelbuild#16912.

PiperOrigin-RevId: 493175302
Change-Id: I7a34c988265f3a3d94f1720a7fc7ad501eb44247

src/main/java/com/google/devtools/build/lib/analysis/config/BuildConfigurationValue.java

@@ -17,7 +17,6 @@
 import com.google.common.annotations.VisibleForTesting;
 import com.google.common.base.Suppliers;
 import com.google.common.collect.ImmutableCollection;
-import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.ImmutableSortedMap;
@@ -746,7 +745,7 @@ public int analysisTestingDepsLimit() {
   }
 
   public List<Label> getActionListeners() {
-    return options.actionListeners == null ? ImmutableList.of() : options.actionListeners;
+    return options.actionListeners;
   }
 
   /**

src/main/java/com/google/devtools/build/lib/analysis/config/ExecutionTransitionFactory.java

@@ -129,7 +129,7 @@ private static BuildOptions transitionImpl(BuildOptionsView options, Label execu
       coreOptions.isHost = false;
       coreOptions.isExec = true;
       // Disable extra actions
-      coreOptions.actionListeners = null;
+      coreOptions.actionListeners = ImmutableList.of();
 
       // Then set the target to the saved execution platform if there is one.
       PlatformOptions platformOptions = execOptions.get(PlatformOptions.class);