Fix -Wnonnull warning in folly/

Summary: libc++ 21 enables stricter `-Wnonnull` diagnostics. `std::string::append` is annotated `nonnull`, so passing a potentially-null pointer is a hard error even when guarded by a null check on an alias. Fix by passing the already-checked local variable `c` instead of the original parameter `value`.

Reviewed By: lexprfuncall

Differential Revision: D94578921

fbshipit-source-id: 15b9b4e87ccf4d8a477672d70eb329bdcb0724f9

Author: yuxuanchen1997

third-party/folly/src/folly/Conv.h

@@ -431,9 +431,8 @@ typename std::enable_if<
 toAppend(Src value, Tgt* result) {
   // Treat null pointers like an empty string, as in:
   // operator<<(std::ostream&, const char*).
-  const char* c = value;
-  if (c) {
-    result->append(value);
+  if (const char* c = value) {
+    result->append(c);
   }
 }