scripts: coredump: Fix loading coredumps when thread info is enabled.

When loading a coredump with an an elf built using
'CONFIG_DEBUG_THREAD_INFO=y', gdbstubs assumes that the thread info
memoryblock populated by 'CONFIG_DEBUG_COREDUMP_MEMORY_DUMP_THREADS=y'
will be present in the coredump. This is not always true and causes an
error, and a failure to load the coredump. Add a default value for the
threads_metadata variable in CoredumpLogFile which can be used to detect
when the memory block is not present. This allows the coredump to load
successfully.

Signed-off-by: Félix Turgeon <felixturgeon@meta.com>