system/log: Add protection against nullptr string parameters

As opposed to printf("%s", nullptr); log::info("{}", nullptr) will
cause a segfault at runtime. This change adds swaps nullptr
string parameters in the code and replaces them with the
"(nullptr)" string.

Bug: 305066880
Test: atest libbluetooth_log_test
Flag: EXEMPT, log change
Change-Id: I9acbcc4dac2c2b12b2ff888805738a088513d45a