[libc] Make 'printf' converter output "(null)" instead of "null"

Summary:
Currently we print `null` for the null pointer in a `%s` expression.
Although it's not defined by the standard, other implementations choose
to use `(null)` to indicate this. We also currently print `(nullptr)` so
I think it's more consistent to use parens in both cases.

libc/src/stdio/printf_core/string_converter.h

@@ -25,7 +25,7 @@ LIBC_INLINE int convert_string(Writer *writer, const FormatSection &to_conv) {
 
 #ifndef LIBC_COPT_PRINTF_NO_NULLPTR_CHECKS
   if (str_ptr == nullptr) {
-    str_ptr = "null";
+    str_ptr = "(null)";
   }
 #endif // LIBC_COPT_PRINTF_NO_NULLPTR_CHECKS
 

libc/test/src/stdio/sprintf_test.cpp

@@ -121,8 +121,8 @@ TEST(LlvmLibcSPrintfTest, StringConv) {
 
 #ifndef LIBC_COPT_PRINTF_NO_NULLPTR_CHECKS
   written = LIBC_NAMESPACE::sprintf(buff, "%s", nullptr);
-  EXPECT_EQ(written, 4);
-  ASSERT_STREQ(buff, "null");
+  EXPECT_EQ(written, 6);
+  ASSERT_STREQ(buff, "(null)");
 #endif // LIBC_COPT_PRINTF_NO_NULLPTR_CHECKS
 }