Introduce non-explicit empty constructor in C++

[mikir]

Currently, Zserio objects in C++ do have the following default constructor:

explicit ZserioObject(const allocator_type& allocator = allocator_type()) noexcept;

This prevents calling of empty constructor implicitly. Example:

ZserioObject zserioObject = {};

This could be potentionally cumbersome for example if ZserioObject is passed as parameter.

The solution could be to split this constructor as follows:

ZserioObject() noexcept;
explicit ZserioObject(const allocator_type& allocator) noexcept;

[Mi-La]

1. We have split single constructor to empty constructor and constructor with allocator_type
2. Field constructor was improved to support {} - i.e. default template parameters were added
   • construction with {} cannot work for structures with single field since the constructor is ambiguous (ambiguity with copy constructor...)
     const TestStruct testStruct = TestStruct(10, {}, "sample", {});
3. Note that when structure has optional fields and {} is used as a field constructor parameter, it DOES NOT mean that optional field is not present (NullOpt) but empty constructor is used to create optional field.

   struct TestStruct
   {
       string strFiedl;
       optional uint32 u32Field;
   };
   

   const TestStruct testStructWithDefaultOptional = TestStruct("sample", {}); // {} does not mean NullOpt!
   const TestStruct testStructWithDefaultOptional = TestStruct("sample", uint32_t()); // same as above
   // vs.
   const TestStruct testStructWithoutOptional = TestStruct("sample", zserio::NullOpt);