Fix conversion compiler warnings

Using a release build with GCC 14.2.0 and -Wextra -Wconversion -Wall.
The generated binaries are not changed by this commit.

taglib/fileref.cpp

@@ -321,7 +321,7 @@ public:
     return !file || !file->isValid();
   }
 
-  bool isNullWithDebugMessage(const String &methodName) const
+  bool isNullWithDebugMessage([[maybe_unused]] const String &methodName) const
   {
     if(isNull()) {
       debug("FileRef::" + methodName + "() - Called without a valid file.");