pgsql: pg_stash_advice: Fix failure to reload empty advice string.

pg_stash_advice: Fix failure to reload empty advice string.

There's not really any good reason to store an empty advice string
in an advice stash, but it's entirely possible that some people might
do it by accident. If they do that without this fix, restarting the
system will result in failure while reloading pg_stash_advice.tsv.

The root cause is that I (rhaas) did not think clearly enough when
defining pgsa_next_tsv_field(). After returning each field on the
line, it set *cursor to the start of the next field, and after
returning all of them, it set *cursor to the trailing NUL byte. But
when the last field on the line is empty, this is ambiguous: if we're
pointing at the trailing NUL byte, it could be either because it's the
start of the zero-length field or because we're out of fields. To
remove the ambiguity, this commit decides that *cursor will instead
be set to NULL when there are no more fields.

Reported-by: Noah Misch <noah(at)leadboat(dot)com>
Discussion: https://postgr.es/m/20260827171830.68.noahmisch@microsoft.com
Backpatch-through: 19

Branch
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REL_19_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/21b2808536859f1ffe257088df0f6f09edfd96a1

Modified Files
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contrib/pg_stash_advice/stashpersist.c | 33 ++++++++++++++++++++------------
contrib/pg_stash_advice/t/001_persist.pl | 18 +++++++++--------
2 files changed, 31 insertions(+), 20 deletions(-)