Fix incorrect nullability annotation of absl::Cord::InlineRep::set_data().

It is called with `(nullptr, 0)` by `absl::Cord` constructor from
`absl::string_view(nullptr, 0)`.

The implementation works for `(nullptr, 0)`.

PiperOrigin-RevId: 802213857
Change-Id: Ibb9b9dae620262ebee8aa3347c4177e7a3da52c0

Author: QrczakMK

absl/strings/cord.cc

@@ -161,8 +161,10 @@ static CordRep* absl_nonnull CordRepFromString(std::string&& src) {
 // --------------------------------------------------------------------
 // Cord::InlineRep functions
 
-inline void Cord::InlineRep::set_data(const char* absl_nonnull data, size_t n) {
+inline void Cord::InlineRep::set_data(const char* absl_nullable data,
+                                      size_t n) {
   static_assert(kMaxInline == 15, "set_data is hard-coded for a length of 15");
+  assert(data != nullptr || n == 0);
   data_.set_inline_data(data, n);
 }
 

absl/strings/cord.h

@@ -921,7 +921,7 @@ class Cord {
     // Returns nullptr if holding pointer
     const char* absl_nullable data() const;
     // Discards pointer, if any
-    void set_data(const char* absl_nonnull data, size_t n);
+    void set_data(const char* absl_nullable data, size_t n);
     char* absl_nonnull set_data(size_t n);  // Write data to the result
     // Returns nullptr if holding bytes
     absl::cord_internal::CordRep* absl_nullable tree() const;