Create index with nullable parts gets them not nullable

[Mons]

Consider the following snippet

box.cfg{}
box.schema.space.create('test', {if_not_exists = true})
box.space.test:format({
    {name = 'id', type = 'number'},
    {name = 'data', type = 'string', is_nullable = true},
})
box.space.test:create_index('primary',{if_not_exists = true})

box.space.test:create_index('sec1', {
    parts = { 2, 'string' },
    if_not_exists = true,
})

box.space.test:create_index('sec2', {
    parts = { 1, 'number', 2, 'string' },
    if_not_exists = true,
})

box.space.test:create_index('sec3', {
    parts = { 'id', 'data' },
    if_not_exists = true,
})

print("first part", box.space.test.index.sec1.parts[1].is_nullable and "correct" or "not correct")
print("second part", box.space.test.index.sec2.parts[2].is_nullable and "correct" or "not correct")
print("second part, named", box.space.test.index.sec3.parts[2].is_nullable and "correct" or "not correct")

require "console".start()
os.exit()

output for 2.10 would be:

first part	correct
second part	not correct
second part, named	correct

We have a space with a nullable field.
If we set this field in composite index, then in some cases in the index field will be not nullable.
This is a regression, in 1.10 everything is ok