Short-circuit deleted package retrieval if list is empty.

--
MOS_MIGRATED_REVID=139127910

src/main/java/com/google/devtools/build/lib/pkgcache/PackageCacheOptions.java

diff --git a/src/main/java/com/google/devtools/build/lib/pkgcache/PackageCacheOptions.java b/src/main/java/com/google/devtools/build/lib/pkgcache/PackageCacheOptions.java
index f22fceb..8237e84 100644
--- a/src/main/java/com/google/devtools/build/lib/pkgcache/PackageCacheOptions.java
+++ b/src/main/java/com/google/devtools/build/lib/pkgcache/PackageCacheOptions.java
@@ -166,7 +166,7 @@
   }
 
   public ImmutableSet<PackageIdentifier> getDeletedPackages() {
-    if (deletedPackages == null) {
+    if (deletedPackages == null || deletedPackages.isEmpty()) {
       return ImmutableSet.of();
     }
     ImmutableSet.Builder<PackageIdentifier> newDeletedPackages = ImmutableSet.builder();