MDEV-30732 : wsrep_store_key_val_for_row() may invoke memcpy() on nullptr

Problem was that row_mysql_read_blob_ref can return NULL
in case when blob datatype is used in a key and its real
value is NULL. This NULL pointer is then used in memcpy
function in wsrep_store_key_val_for_row. However,
memcpy is defined so that argument 2 must not be NULL.

Fixed by adding conditions before memcpy functions so
that argument 2 is always non NULL.

Additional fixes after review
- Removed unnecessary copying key data from one buffer to another.
Use original key data buffer as input and temporary buffer as output.
Extra output buffer is needed because strnxfrm might expand input buffer
contents.
- Removed unnecessary initialization of variables and move
declaration where first time needed.
- Removed unnecessary intitialization of temporary buffer because
we already keep track actual filled length.
- Remove unneccessary extra call to charset->strnxfrm

Author: janlindstrom