Merge bitcoin/bitcoin#26708: clang-tidy: Fix modernize-use-nullptr in headers

adb7dba clang-tidy: Fix `modernize-use-nullptr` in headers (Hennadii Stepanov)

Pull request description:

  Split from bitcoin/bitcoin#26705 as was requested in bitcoin/bitcoin#26705 (comment).

  To test this PR, consider applying a diff as follows:
  ```diff
  --- a/src/.clang-tidy
  +++ b/src/.clang-tidy
  @@ -12,17 +12,9 @@ readability-redundant-declaration,
   readability-redundant-string-init,
   '
   WarningsAsErrors: '
  -bugprone-argument-comment,
  -bugprone-use-after-move,
  -misc-unused-using-decls,
  -modernize-use-default-member-init,
   modernize-use-nullptr,
  -performance-for-range-copy,
  -performance-move-const-arg,
  -performance-unnecessary-copy-initialization,
  -readability-redundant-declaration,
  -readability-redundant-string-init,
   '
   CheckOptions:
    - key: performance-move-const-arg.CheckTriviallyCopyableMove
      value: false
  +HeaderFilterRegex: '.'
  ```

ACKs for top commit:
  john-moffett:
    ACK adb7dba

Tree-SHA512: 67241fb212d837157a0a26f0d59e7f30a9d270d5b0ebfeb6ad9631e460fc7fba8c9a9dcd4c0520789353f68025a9f090f40f17176472a93cce1411e6d56f930b

Author: MarcoFalke

src/tinyformat.h

@@ -508,9 +508,9 @@ class FormatArg
 {
     public:
         FormatArg()
-            : m_value(NULL),
-            m_formatImpl(NULL),
-            m_toIntImpl(NULL)
+            : m_value(nullptr),
+            m_formatImpl(nullptr),
+            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