Use non-null return in CfInstructionDesugaring#compute implementation

Change-Id: If87e938458020b9c396b67a3dbd30d7628b4435f
diff --git a/src/main/java/com/android/tools/r8/ir/desugar/desugaredlibrary/retargeter/CfToCfDesugaredLibraryRetargeter.java b/src/main/java/com/android/tools/r8/ir/desugar/desugaredlibrary/retargeter/CfToCfDesugaredLibraryRetargeter.java
index 48a29d5..14930ea 100644
--- a/src/main/java/com/android/tools/r8/ir/desugar/desugaredlibrary/retargeter/CfToCfDesugaredLibraryRetargeter.java
+++ b/src/main/java/com/android/tools/r8/ir/desugar/desugaredlibrary/retargeter/CfToCfDesugaredLibraryRetargeter.java
@@ -77,7 +77,7 @@
               })
           .build();
     }
-    return null;
+    return DesugarDescription.nothing();
   }
 
   private DesugarDescription computeStaticFieldGetDescription(