Fix SIGSEGV when --null is empty or whitespace-only

[vulragrag-star]

Fix for issue #269.

--null with an empty or whitespace-only value makes trim_quoted_str() return NULL, and sstrndup() then crashes. Guard that case the same way config.c already does (str ? sstrndup(...) : ...) and store an empty nullstr instead.

# before: SIGSEGV
printf 'a\tb\n1\t\n' | ./pspg --tsv --null='' -X

# after: renders table, exit 0
printf 'a\tb\n1\t\n' | ./pspg --tsv --null='' -X
printf 'a\tb\n1\t\n' | ./pspg --tsv --null='   ' -X
./pspg --null='' </dev/null   # "No data", no crash

Also rebuilt with ./configure && make (same as the Linux workflow).