Correct nullability annotations on MakeNan

Neither GCC nor Clang's __builtin_nan(), nor glibc's nan(), accept null pointers.

PiperOrigin-RevId: 595767225
Change-Id: I4cddd1cafd0c9e83a823ec68386f10ce077c6b4c

absl/strings/charconv.cc

diff --git a/absl/strings/charconv.cc b/absl/strings/charconv.cc
index eda6d50..0c9227f 100644
--- a/absl/strings/charconv.cc
+++ b/absl/strings/charconv.cc
@@ -120,7 +120,7 @@
   // Parsing a smaller N will produce something finite.
   static constexpr int kEiselLemireMaxExclusiveExp10 = 309;
 
-  static double MakeNan(absl::Nullable<const char*> tagp) {
+  static double MakeNan(absl::Nonnull<const char*> tagp) {
 #if ABSL_HAVE_BUILTIN(__builtin_nan)
     // Use __builtin_nan() if available since it has a fix for
     // https://bugs.llvm.org/show_bug.cgi?id=37778
@@ -193,7 +193,7 @@
   static constexpr int kEiselLemireMinInclusiveExp10 = -46 - 18;
   static constexpr int kEiselLemireMaxExclusiveExp10 = 39;
 
-  static float MakeNan(absl::Nullable<const char*> tagp) {
+  static float MakeNan(absl::Nonnull<const char*> tagp) {
 #if ABSL_HAVE_BUILTIN(__builtin_nanf)
     // Use __builtin_nanf() if available since it has a fix for
     // https://bugs.llvm.org/show_bug.cgi?id=37778