[cpp-restsdk] Fix ModelBase::fromJson(const web::json::value&, int64_t&) bug which incorrectly returns zero

[shayan-eftekhari]

fix #8450
The issue and solution is completely described by @keysight-diana-belmega who reported the issue (look #8450).

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@ravinikam @stkrwork @etherealjoy @MartinDelille @muttleyxd

[wing328]

Thanks for the fix, which looks good to me but I've not tested it locally.