Fix modernize-use-nullptr clang-tidy issues

This CL also includes a few exceptions to the android-cloexec-open
check in profile_assistant_test.cc, which are required to make
clang-tidy run without any errors.

Bug: 213953102
Test: m tidy-art
Change-Id: I4d99b51a2d2c1837ef350d44965d386738b74065
3 files changed