clang-tidy: Fix modernize-use-nullptr in headers

https://clang.llvm.org/extra/clang-tidy/checks/modernize/use-nullptr.html

src/tinyformat.h

@@ -508,9 +508,9 @@ class FormatArg
 {
     public:
         FormatArg()
-            : m_value(NULL),
+            : m_value(nullptr),
-            m_formatImpl(NULL),
+            m_formatImpl(nullptr),
-            m_toIntImpl(NULL)
+            m_toIntImpl(nullptr)
         { }
 
         template<typename T>
@@ -1005,7 +1005,8 @@ class FormatListN : public FormatList
 // Special 0-arg version - MSVC says zero-sized C array in struct is nonstandard
 template<> class FormatListN<0> : public FormatList
 {
-    public: FormatListN() : FormatList(0, 0) {}
+public:
+    FormatListN() : FormatList(nullptr, 0) {}
 };
 
 } // namespace detail