datetime: fix values of unspecified fields in dt.parse()

datetime object can be created using `datetime.new()` and
`datetime.parse()`. `datetime.new()` sets values of unspecified
fields to appropriate values of fields in a Unix time
(00:00:00 UTC on 1 January 1971, Thursday), see commit
43e10ed3 ("build, lua: built-in module datetime"):

```
tarantool> datetime.new()
---
- 1970-01-01T00:00:00Z
...

tarantool> datetime.new():totable()
---
- sec: 0
  min: 0
  yday: 1
  day: 1
  nsec: 0
  isdst: false
  wday: 5
  tzoffset: 0
  month: 1
  year: 1970
  hour: 0
```

The function `datetime.parse` converts an input string with the
date and time information into a datetime object. When fields are
not specified in a input string their values becomes undefined:

```
tarantool> dt.parse('01:01:01', {format ='%H:%M:%S'}):totable()
---
- sec: -59
  min: -58
  yday: 366
  day: 31
  nsec: 0
  isdst: false
  wday: 1
  tzoffset: 0
  month: 12
  year: 0
  hour: -22
...

tarantool>
```

The commit fixes aforementioned behaviour of `datetime.parse()`
by setting values for fields not specified by user to values of
appropriate fields of Unix time.

NO_DOC=bugfix

Fixes #8588

(cherry picked from commit 9ac56a1222e916951010091fbdbfc497f4a4971d)