Convert an if-condition from Automator-powered AppleScript to pure AppleScript

Here is an AppleScript script to make it possible double-click files in Finder so that they will be opened in Vim, by Normen Hansen: https://github.com/normen/vim-macos-scripts/blob/master/open-file-in-vim.applescript (it must be used from Automator).

I try to simplify it and also to make it possible to use it without Automator. To use my own version, you only need to save it as application instead, like this:

osacompile -o open-with-vim.app open-with-vim.applescript

and then copy it to /Applications and set your .txt files to be opened using this app.

Here it is, it alsmost works:

-- https://github.com/normen/vim-macos-scripts/blob/master/open-file-in-vim.applescript
-- opens Vim with file or file list
-- sets current folder to parent folder of first file

on open theFiles
  set command to {}
  if input is null or input is {} or ((item 1 in input) as string) is "" then
    -- no files, open vim without parameters
    set end of command to "vim;exit"
  else
    set firstFile to (item 1 of theFiles) as string
    tell application "Finder" to set pathFolderParent to quoted form of (POSIX path of ((folder of item firstFile) as string))
    set end of command to "cd" & space & (pathFolderParent as string)
    set end of command to ";hx" -- e.g. vi or any other command-line text editor
    set fileList to {}
    repeat with i from 1 to (theFiles count)
      set end of fileList to space & quoted form of (POSIX path of (item i of theFiles as string))
    end repeat
    set end of command to (fileList as string)
    set end of command to ";exit" -- if Terminal > Settings > Profiles > Shell > When the shell exits != Don't close the window
  end if
  set command to command as string
  set myTab to null
  tell application "Terminal"
    if it is not running then
      set myTab to (do script command in window 1)
    else
      set myTab to (do script command)
    end if
    activate
  end tell
  return
end open

The only problem is the if block in the very beginning:

if input is null or input is {} or ((item 1 in input) as string) is "" then

What is wrong there? How to make it work? (Assuming it already works fine if AppleScript is used using Automator.)