pgsql: Make GetConfigOption/GetConfigOptionResetString return "" for NU

Make GetConfigOption/GetConfigOptionResetString return "" for NULL.

As per the preceding commit, GUC APIs generally expose NULL-valued
string variables as empty strings.  Extend that policy to
GetConfigOption() and GetConfigOptionResetString(), eliminating
a crash hazard for unwary callers, as well as a fundamental
ambiguity in GetConfigOption()'s API.

No back-patch, since this is an API change and conceivably somebody
somewhere is depending on this corner case.

Xing Guo, Aleksander Alekseev, Tom Lane

Discussion: https://postgr.es/m/CACpMh+AyDx5YUpPaAgzVwC1d8zfOL4JoD-uyFDnNSa1z0EsDQQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0bc726d95a305ca923b1f284159f40f0d5cf5725

Modified Files
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src/backend/utils/misc/guc.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)