Fix nullable string example failure on gcc7

[bdeng-xt]

The nullable string array example(nullable_attribute.cc) fails on CentOS7/gcc7.3 with an incomplete query. Finally I found that it is not a bug from gcc7. The bug for calculating max_size_validity caused that problem. When we add three attributes a1,a2,a3, the unordered_map buffer still keep in that order gcc4.8, clang, and vc2015. But in gcc 7, a2 becomes the first when we try to iterate. The incomplete query is not found when we use gcc4.8, clang or vc2015, since a1 is a fixed size attribute and a1 still shows to be the first in gcc4.8 and other compilers. If our example use more attributes, the incomplete query could also happen for gcc4.8, clang and vc2015. This incomplete query can also happen for other kinds of variable size attributes(such as vector). In this branch, I also fixed some warnings from gcc compiler when we provided unused parameters for default constructors and assign operators.

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TYPE: BUG
DESC: The bug for calculating max_size_validity for var_size attribute caused incomplete query

[joe-maley]

Great fix!

FYI, Windows CI is failing on:

D:\a\1\s\examples\cpp_api\nullable_attribute.cc(185): warning C4267: '=': conversion from 'size_t' to 'int', possible loss of data