Handle workflow_dispatch trigger

[Aeris1One]

When a workflow is manually triggered, the commit sha is set to undefined.
This isn't handled properly by this Action which try to run git diff --name-only --diff-filter=AMR -M100% <revisioncommit> undefined

A sample error log would be :

Run swznd/sftp-deploy@master
  with:
    host: ***
    port: ***
    user: ***
    password: ***
    ignore: .github/**,.gitignore,**/.gitignore,**/.git/**
Connected. Current Working Directory: /
Remote Revision: 
Remote revision empty, get from initial commit
/usr/bin/git hash-object -t tree /dev/null
4b825dc642cb6eb9a060e54bf8d69288fbee4904
Comparing 4b825dc642cb6eb9a060e54bf8d69288fbee4904..undefined
/usr/bin/git diff --name-only --diff-filter=AMR -M100% 4b825dc642cb6eb9a060e54bf8d69288fbee4904 undefined
fatal: ambiguous argument 'undefined': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
Error: The process '/usr/bin/git' failed with exit code 128