Don't try to translate NULL in GetConfigOptionByNum().

Noticed via -fsanitize=undefined. Introduced when a few columns in
GetConfigOptionByNum() / pg_settings started to be translated in 72be8c29 /
PG 12.

Backpatch to all affected branches, for the same reasons as 46ab07ffda9.

Discussion: https://postgr.es/m/20220323173537.ll7klrglnp4gn2um@alap3.anarazel.de
Backpatch: 12-

src/backend/utils/misc/guc.c

@@ -9601,7 +9601,7 @@ GetConfigOptionByNum(int varnum, const char **values, bool *noshow)
 	values[4] = _(conf->short_desc);
 
 	/* extra_desc */
-	values[5] = _(conf->long_desc);
+	values[5] = conf->long_desc != NULL ? _(conf->long_desc) : NULL;
 
 	/* context */
 	values[6] = GucContext_Names[conf->context];