pgsql: Be more wary about NULL values for GUC string variables.

Be more wary about NULL values for GUC string variables.

get_explain_guc_options() crashed if a string GUC marked GUC_EXPLAIN
has a NULL boot_val.  Nosing around found a couple of other places
that seemed insufficiently cautious about NULL string values, although
those are likely unreachable in practice.  Add some commentary
defining the expectations for NULL values of string variables,
in hopes of forestalling future additions of more such bugs.

Xing Guo, Aleksander Alekseev, Tom Lane

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

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6e1cca5113487bb9ef4daa1bb2833c6333466410

Modified Files
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src/backend/utils/misc/guc.c   |  3 ++-
src/include/utils/guc_tables.h | 10 ++++++++++
2 files changed, 12 insertions(+), 1 deletion(-)