box: don't check return values of some cfg_gets() for NULL

Sometimes the return value of cfg_gets() is checked for NULL, and sometimes
not. Actually this is intended, although a bit confusing. If an option can
have a nil value, it must be checked for NULL, but if it can't be nil,
there is no sense in it. The nil value can be assigned only by default, it
cannot be set via box.cfg{}.

This patch removes the NULL checks for cfg_gets("election_mode") and
cfg_gets("election_fencing_mode") because they are not nil by default.
All other non-nil options (e.g. cfg_gets("bootstrap_strategy")) are
already implemented without the NULL checks.

Follow-up tarantool/security#75

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring

(cherry picked from commit 5a2dc43c)