src: apply clang-tidy readability-delete-null-pointer
see: https://isocpp.org/wiki/faq/freestore-mgmt#delete-handles-null and https://clang.llvm.org/extra/clang-tidy/checks/readability-delete-null-pointer.html.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
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