default entry_maker not set properly

[mnowotnik]

Description

Not setting entry_maker explicitly should fallback gracefully, Currently, it doesn't work, because
it's being set to make_entry.gen_from_string and make_entry.gen_from_string is entry_maker factory.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Ran a script:

  pickers.new(opts, {
    finder = finders.new_dynamic({fn=function(_)
      return {"a","b","c"}
    end}):find()

Configuration:

• Neovim version 0.7.2
• Operating system and version: Ubuntu 20.04

Checklist:

• My code follows the style guidelines of this project (stylua)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (lua annotations)