Use NullSafeStringView for const char* args to absl::StrCat, treating null pointers as ""

Fixes #1167

PiperOrigin-RevId: 449328725
Change-Id: I813785db77b94efa49eeeff4c93449334c380935

absl/strings/str_cat.h

@@ -251,7 +251,8 @@ class AlphaNum {
       const strings_internal::AlphaNumBuffer<size>& buf)
       : piece_(&buf.data[0], buf.size) {}
 
-  AlphaNum(const char* c_str) : piece_(c_str) {}  // NOLINT(runtime/explicit)
+  AlphaNum(const char* c_str)                     // NOLINT(runtime/explicit)
+      : piece_(NullSafeStringView(c_str)) {}      // NOLINT(runtime/explicit)
   AlphaNum(absl::string_view pc) : piece_(pc) {}  // NOLINT(runtime/explicit)
 
   template <typename Allocator>

absl/strings/str_cat_test.cc

@@ -210,6 +210,11 @@ TEST(StrCat, CornerCases) {
   EXPECT_EQ(result, "");
 }
 
+TEST(StrCat, NullConstCharPtr) {
+  const char* null = nullptr;
+  EXPECT_EQ(absl::StrCat("mon", null, "key"), "monkey");
+}
+
 // A minimal allocator that uses malloc().
 template <typename T>
 struct Mallocator {