Allow checking for backports of classes which are not found at all

E.g. sun.misc.Unsafe has no definition at compile time.

Bug: b/232073181
Fixes: b/298627017
Change-Id: I996ab087d7dc5e779e665b87edd01d1e21c1b141

src/main/java/com/android/tools/r8/ir/desugar/BackportedMethodRewriter.java

diff --git a/src/main/java/com/android/tools/r8/ir/desugar/BackportedMethodRewriter.java b/src/main/java/com/android/tools/r8/ir/desugar/BackportedMethodRewriter.java
index 38ce1d8..3c8315b 100644
--- a/src/main/java/com/android/tools/r8/ir/desugar/BackportedMethodRewriter.java
+++ b/src/main/java/com/android/tools/r8/ir/desugar/BackportedMethodRewriter.java
@@ -187,7 +187,7 @@
           appView
               .contextIndependentDefinitionForWithResolutionResult(provider.method.holder)
               .toSingleClassWithProgramOverLibrary();
-      if (!clazz.isProgramDefinition()) {
+      if (clazz == null || !clazz.isProgramDefinition()) {
         appView
             .reporter()
             .warning(