Update last updated time if there are no available updates

[jnooree]

Standards checklist:

• The PR title is descriptive.
• The PR doesn't replicate another PR which is already open.
• I have read the contribution guide and followed all the instructions.
• The code follows the code style guide detailed in the wiki.
• The code is mine or it's from somewhere with an MIT-compatible license.
• The code is efficient, to the best of my ability, and does not waste computer resources.
• The code is stable and I have tested it myself, to the best of my abilities.

Changes:

• Call update_last_updated_file if there are no available updates.

Other comments:

The :omz:update frequency configuration does not have to be exact "update" days; it is sufficient to check every frequency day.

This change is proposed because the is_update_available function accesses the network, so it consumes a significant amount of time on every startup until there are some upstream updates.