Update to modern style cast

[bmagistro]

When compiling with clang 19.1.7 on Alma with -Werror and a number of other flags (we run with most warnings enabled), the lines were flagged with error: use of old-style cast [-Werror, -Wold-style-cast]. It is unclear if this was just missed or cast style was not updated. Additionally, 0 was switched to nullptr for improved clarity in these lines.

The updates here were found in conjunction with boostorg/function#55.

Env:
Alma 9.6
Boost 1.88.0 via cmake
clang 19.1.7

[vprus]

Thanks for the PR.

Tehnically, the use of nullptr is a C++11 feature, but apparently shared_ptr now requires a C++11 compiler, so program_options would not build with C++03 anyway.