Fix don't quote prompt when quote_char is empty string

I already have a keymap to quote the prompt, so my current workflow is:
type query-> Quote prompt -> Start adding options

However I would like to create keymaps for some common options I use,
currently this not possible because when `quote_char` is emtpy, it still adds
backslashes before each character in prompt, and this breaks the search pattern

Usecase is for chaining mulitple `rg` options without, quoting
the prompt mulitple times, which is not the expected behavior when
`quote_char` is empty string.

E.g Consider this keymap

```lua
["<C-;>"] = require("telescope-live-grep-args.actions").quote_prompt {
        postfix = " --word-regexp ",
        quote_char = "",
    }
```

And I search for `tab`, then pressing `<C-;>`:

Before:

```
\"\t\a\b\"\ --word-regexp
```

After:

```
"tab" --word-regexp
```

lua/telescope-live-grep-args/actions/quote_prompt.lua

@@ -21,7 +21,11 @@ return function (opts)
     if opts.trim then
       prompt = vim.trim(prompt)
     end
-    prompt = helpers.quote(prompt, { quote_char =  opts.quote_char }) .. opts.postfix
+    if opts.quote_char == "" then
+      prompt = prompt .. opts.postfix
+    else
+      prompt = helpers.quote(prompt, { quote_char =  opts.quote_char }) .. opts.postfix
+    end
     picker:set_prompt(prompt)
   end
 end