Avoid assertion in bridges-generation for light classes

^KT-11656

compiler/backend/src/org/jetbrains/kotlin/codegen/builtinSpecialBridges.kt

@@ -153,7 +153,7 @@ private fun <Signature> findSuperImplementationForStubDelegation(
     state: GenerationState,
     signatureByDescriptor: (FunctionDescriptor) -> Signature
 ): FunctionDescriptor? {
-    val implementation = findConcreteSuperDeclaration(DescriptorBasedFunctionHandleForJvm(function, state))
+    val implementation = findConcreteSuperDeclaration(DescriptorBasedFunctionHandleForJvm(function, state)) ?: return null
 
     // Implementation from interface will be generated by common mechanism
     if (!implementation.mayBeUsedAsSuperImplementation) return null