[cpp-rest] Fixes segfault for nullable strings

[winrid]

Fixes #22404

Was seeing segfaults on nullable fields that were actually set to null at runtime.
Switches to boost optional, which seems to be the best way to go, or I can add guards instead of we prefer that.

@ravinikam @stkrwork @etherealjoy @MartinDelille @muttleyxd @aminya

[muttleyxd]

I like the idea, however we have std::optional since C++17, so I'd suggest to use that - it's been in standard for 8 years and it doesn't require external libraries

[winrid]

@muttleyxd The sdk's min compile target listed in cmake-lists is 11, should we update it?

Looks like sometimes cpp11 will work, unless you use anyOf... or now optional...

FYI the library already depends on boost, and the compile time for this header should be the same as std. I wasn't sure how far back we really want to go in terms of support so I did the safe thing. I guess I will change it.

[muttleyxd]

Oh, I haven't looked into this generator recently (in last few years 😅), I guess if we keep it at C++11 and Boost is already there, then it's fine

[winrid]

it's ok :) I will do whatever people want me to do. Personally I don't think consumers of my sdk will be using anything older than 17...

[wing328]

if no further feedback, i'll merge this PR tomorrow or Wed.

thanks again for the contribution