Fix E474 error in minimap_buffer_enter_handler when opening minimap buffer without a file

[kuangyujing]

Check list

• I have read through the README (especially F.A.Q section)
• I have searched through the existing issues or pull requests
• I have verified all existing unit tests pass (See the README on how to run)
• I have added new tests if appropriate
• I have performed a self-review of my code and commented hard-to-understand areas
• I have made corresponding changes to the documentation (when necessary)

Description

This commit addresses an issue where the minimap plugin would throw an "E474: Invalid argument" error when Vim is opened without a file and the minimap buffer is focused. The error was caused by attempting to move the cursor to an invalid position.

The fix adds a condition to s:minimap_buffer_enter_handler() to ensure s:last_pos is not empty or less than or equal to zero before calling cursor().

This resolves the issue by preventing the cursor from moving to an invalid position.

Issue: #199

Type of change

• Bug fix
• New feature
• Improvement of existing features
• Refactor
• Breaking change
• Documentation change
• CI / CD

Test environment

• OS
  • Linux
  • Mac OS X
  • Windows
  • Others:
• Vim
  • Neovim:
  • Vim: 9.1

[kuangyujing]

Testify result:

* t/git_tests.vim
√ Git - Multiline diff
√ Git - Short file diff
√ Git - Long file diff
√ Git - Diff spans minimap lines
√ Git - Extras in string are ignored

* t/utility_tests.vim
√ Minimap conversion math works as expected
√ Opening minimap results in only one call to update_minimap

* t/search_tests.vim
√ Search setup
√ Search - Beginning of file
√ Search - Beginning of line
√ Search - End of line
√ Search - Multi match per line
√ Search - Spanning a line
√ Search - Many results
√ Search - Long Match
√ Search - History
√ Search tear down

* Test Summary
* ------------
* All tests successful
* Tests=17
* Result: PASS