[libutils]  Modernize codebase by replacing NULL with nullptr

Fixes -Wzero-as-null-pointer-constant warning.

Test: m
Bug: 68236239
Change-Id: I5e89ec8c42151875439d2656475a8739ab9cb7dc
diff --git a/libutils/String16.cpp b/libutils/String16.cpp
index 84d53dd..f820b8b 100644
--- a/libutils/String16.cpp
+++ b/libutils/String16.cpp
@@ -74,7 +74,7 @@
 }
 
 String16::String16(StaticLinkage)
-    : mString(0)
+    : mString(nullptr)
 {
     // this constructor is used when we can't rely on the static-initializers
     // having run. In this case we always allocate an empty string. It's less
@@ -336,7 +336,7 @@
 {
     const size_t N = size();
     const char16_t* str = string();
-    char16_t* edit = NULL;
+    char16_t* edit = nullptr;
     for (size_t i=0; i<N; i++) {
         const char16_t v = str[i];
         if (v >= 'A' && v <= 'Z') {
@@ -358,7 +358,7 @@
 {
     const size_t N = size();
     const char16_t* str = string();
-    char16_t* edit = NULL;
+    char16_t* edit = nullptr;
     for (size_t i=0; i<N; i++) {
         if (str[i] == replaceThis) {
             if (!edit) {