refactor(Core): apply clang-tidy modernize-use-nullptr (#3819)

src/common/Database/Field.h

@@ -172,7 +172,7 @@ public:
 
         if (data.raw)
             return *reinterpret_cast<int64*>(data.value);
-        return static_cast<int64>(strtol((char*)data.value, NULL, 10));
+        return static_cast<int64>(strtol((char*)data.value, nullptr, 10));
     }
 
     [[nodiscard]] float GetFloat() const
@@ -214,7 +214,7 @@ public:
     [[nodiscard]] char const* GetCString() const
     {
         if (!data.value)
-            return NULL;
+            return nullptr;
 
 #ifdef ACORE_DEBUG
         if (IsNumeric())
@@ -244,7 +244,7 @@ public:
 
     [[nodiscard]] bool IsNull() const
     {
-        return data.value == NULL;
+        return data.value == nullptr;
     }
 
 protected:
@@ -275,7 +275,7 @@ protected:
     void CleanUp()
     {
         delete[] ((char*)data.value);
-        data.value = NULL;
+        data.value = nullptr;
     }
 
     static size_t SizeForType(MYSQL_FIELD* field)

src/common/Database/MySQLConnection.h

@@ -79,7 +79,7 @@ public:
     void CommitTransaction();
     bool ExecuteTransaction(SQLTransaction& transaction);
 
-    operator bool () const { return m_Mysql != NULL; }
+    operator bool () const { return m_Mysql != nullptr; }
     void Ping() { mysql_ping(m_Mysql); }
 
     uint32 GetLastError() { return mysql_errno(m_Mysql); }

src/common/Database/MySQLThreading.h

@@ -35,7 +35,7 @@ public:
 
     static void Library_Init()
     {
-        mysql_library_init(-1, NULL, NULL);
+        mysql_library_init(-1, nullptr, nullptr);
     }
 
     static void Library_End()

src/common/Database/SQLOperation.h

@@ -48,7 +48,7 @@ class MySQLConnection;
 class SQLOperation : public ACE_Method_Request
 {
 public:
-    SQLOperation(): m_conn(NULL) { }
+    SQLOperation(): m_conn(nullptr) { }
     int call() override
     {
         Execute();