[Runtime] Return nullptr instead of NULL in lookupSection stubs.

rdar://problem/36997475

stdlib/public/runtime/ImageInspectionELF.cpp

@@ -146,7 +146,7 @@ int swift::lookupSymbol(const void *address, SymbolInfo *info) {
 // This is only used for backward deployment hooks, which we currently only support for
 // MachO. Add a stub here to make sure it still compiles.
 void *lookupSection(const char *segment, const char *section, size_t *outSize) {
-  return NULL;
+  return nullptr;
 }
 
 #endif // defined(__ELF__)