Fix comparison warnings caused by 54fc4e2

[JensMertelmeyer]

The commit in 54fc4e2 introduces several warnings of "comparison of integer expressions of different signedness: 'unsigned int' and 'int'". Example:

src/./json/json_value.cpp: In instantiation of 'bool Json::InRange(double, T, U) [with T = int; U = unsigned int]':
src/./json/json_value.cpp:687:5:   required from here
src/./json/json_value.cpp:106:30: warning: comparison of integer expressions of different signedness: 'unsigned int' and 'int' [-Wsign-compare]
  106 |          !(static_cast<U>(d) == min && d != integerToDouble(min));
      |            ~~~~~~~~~~~~~~~~~~^~~~~~

This is because maxUInt is unsigned while the 0 literal is not.
I replaced them with 0u and it looks fine again.