tap: fix is_deeply box.NULL corner cases

The tap:is_deeply call used to return inconsistent result
processing an empty tuple and tuple that has no values:
is_deeply({a = box.NULL}, {})  == true
is_deeply({}, {a = box.NULL}) == false
Fixed to return true by default in such case. You may also
set tap.strict = true to change this behaviour.

@TarantoolBot document
Title: tap test new flag 'strict'
In some scenarios it is convenient to distinguish box.NULL and
nil in tap:is(), tap:isnt(), tap:is_deeply() tests.
For example, {result='Success'} and {result='Success', error=None}
are different HTTP responses. You may set t.strict = true to
behave such way.
Example:
t = require('tap').test('123')
t.strict = true
t:is_deeply({a = box.NULL}, {}) -- false

Closes #4125