RELNOTES: Use a default implementation of a progress message, rather than defaulting to null for all SpawnActions.

Right now, all Objc actions, and many others, have no progress message because
the sensible default in AbstractAction is overridden to return null.

--
MOS_MIGRATED_REVID=93930803

src/main/java/com/google/devtools/build/lib/analysis/actions/SpawnAction.java

diff --git a/src/main/java/com/google/devtools/build/lib/analysis/actions/SpawnAction.java b/src/main/java/com/google/devtools/build/lib/analysis/actions/SpawnAction.java
index 724b1a7..839e073 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/actions/SpawnAction.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/actions/SpawnAction.java
@@ -313,7 +313,10 @@
 
   @Override
   protected String getRawProgressMessage() {
-    return progressMessage;
+    if (progressMessage != null) {
+      return progressMessage;
+    }
+    return super.getRawProgressMessage();
   }
 
   @Override