fix(picker.git): handle unnamed buffers in git log pickers

[jiminu]

Description

git_log_file and git_log_line currently pass the empty string returned by nvim_buf_get_name() for unnamed buffers to Git. Since an empty string is truthy in Lua, both the rename-detection command and the main log command run with an empty pathspec and report command failures.

This reads the current buffer name once for file-scoped log pickers and returns an empty finder with a clear warning when the buffer has no file. Repository-wide git_log behavior is unchanged.

This fixes the validation in the picker source so callers do not need to guard every keymap, as proposed in #2599.

Related Issue(s)

Related to #2599.

Testing

• Reproduced both failing Git commands with git_log_file on an unnamed buffer before the change
• Verified git_log_file and git_log_line on unnamed and nofile buffers show Current buffer has no file without a Git command failure
• Verified both pickers still run for a tracked file (README.md)
• Ran git diff --check