implement stringify for std::nullopt_t

[mjerabek]

Description

With CHECK(some_optional == std::nullopt), nullopt was stringified differently than std::optional<T>() (that is, {?} as opposed to { }). That's because std::nullopt is not an optional but a special type.

This MR adds stringification for std::nullopt_t.

[codecov]

Codecov Report

Merging #2740 (a62dcc9) into devel (f24d39e) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            devel    #2740      +/-   ##
==========================================
- Coverage   91.34%   91.32%   -0.01%     
==========================================
  Files         192      192              
  Lines        7850     7850              
==========================================
- Hits         7170     7169       -1     
- Misses        680      681       +1     

[mjerabek]

Btw, is it expected that I didn't have to modify the approval tests? That is, is it expected that ToStringOptional.tests.cpp is not included in them?

[horenmar]

That's what the approvals tag does.