[Refactoring] Skip failed witness decls in "add codable implementation"

When there are problems in the properties in the target type, witness
methods may not be synthesized. Don't try to add such methods.

https://github.com/apple/swift/issues/72387

lib/Refactoring/AddExplicitCodableImplementation.cpp

@@ -133,7 +133,7 @@ class AddCodableContext {
           kind == KnownProtocolKind::Decodable) {
         for (auto requirement : protocol->getProtocolRequirements()) {
           auto witness = conformance->getWitnessDecl(requirement);
-          if (witness->isSynthesized()) {
+          if (witness && witness->isSynthesized()) {
             Printer.printNewline();
             witness->print(Printer, Options);
             Printer.printNewline();