[Runtime] Accept nil in swift_getObjCClassFromMetadata.

Somehow, clang was generating code that accepted nil and returned nil in swift_getObjCClassFromMetadata, but is no longer doing so. Some code relies on this to work, so switch to dyn_cast_or_null to accept nil explicitly.

rdar://74895271

Author: mikeash

stdlib/public/runtime/Metadata.cpp

@@ -1028,13 +1028,13 @@ swift::swift_getObjCClassMetadata(const ClassMetadata *theClass) {
 const ClassMetadata *
 swift::swift_getObjCClassFromMetadata(const Metadata *theMetadata) {
   // Unwrap ObjC class wrappers.
-  if (auto wrapper = dyn_cast<ObjCClassWrapperMetadata>(theMetadata)) {
+  if (auto wrapper = dyn_cast_or_null<ObjCClassWrapperMetadata>(theMetadata)) {
     return wrapper->Class;
   }
 
   // Otherwise, the input should already be a Swift class object.
   auto theClass = cast<ClassMetadata>(theMetadata);
-  assert(theClass->isTypeMetadata());
+  assert(!theClass || theClass->isTypeMetadata());
   return theClass;
 }