coredumpctl: use NULL outputs for COREDUMP existence checks

coredumpctl: use NULL outputs for COREDUMP existence checks

print_list() and print_info() used RETRIEVE() to strndup() the entire
COREDUMP field into a heap-allocated string, only to check whether it
exists.  With sd_journal_set_data_threshold(j, 0) in print_info(),
this copies the full coredump binary (potentially hundreds of MB) to
heap just to print "Storage: journal".

Now that sd_journal_get_data() accepts NULL output pointers, use a
direct NULL/NULL existence check instead.

Co-developed-by: Claude Opus 4.6 <noreply@anthropic.com>