mgr/PyFormatter: prevent segfault on non-printable characters

If PyFormatter::dump_string attempting to format non-printable characters
can result in a nullptr being passed to dump_pyobject, causing a segfault.

It occured in the following case - crash module tried to load the crash entries
containing matedata with non-printable characters.
when these were passed to PyFormatter, the resulting crash prevented ceph-mgr
from starting.

Fix this by attempting to create a python object using PyUnicode_FromString.
If it returns nullptr (indicating a decoding failure), the code now falls
back to decoding using Latin1.
Additionally, add a guard in dump_pyobject to prevent processing nullptr objects,
providing a secondary layer of protection against segfaults.

Fixes: https://tracker.ceph.com/issues/74379
Signed-off-by: Nitzan Mordechai <nmordec@ibm.com>