resolve #18361

Its possible for a dynamic parameter option value to be an empty string
which will cause the following error in the Radix Select component. The
solution is to handle empty strings so that they are not set directly in
the component.


`Uncaught Error: A <Select.Item /> must have a value prop that is not an
empty string. This is because the Select value can be set to an empty
string to clear the selection and show the placeholder.`


```
data "coder_parameter" "radio" {
  name         = "radio"
  display_name = "An example of a radio input"
  description  = "The next parameter supports a single value."
  type         = "string"
  form_type    = "dropdown"
  order        = 1
  default      = ""

  option {
    name = "Empty"
    value = "" 
  }
}

```

Addresses part of the frontend changes needed for
#18516

While a workspace is starting, display the build status and a progress bar.

`coderd` currently does not handle a session token header value of the form `token1, token2`. However, it does handle multiple instances of the token header by simply taking the first. This is the default behaviour of `http.Header.Get`.

So, setting the token header twice causes issues when Coder is behind a proxy that merges duplicate headers, such as [Apache](https://httpd.apache.org/docs/2.4/mod/mod_headers.html#:~:text=list%20of%20values.-,When%20a%20new%20value%20is%20merged%20onto%20an%20existing%20header,format%20specifiers%20have%20been%20processed).


This PR ensures we don't set it twice by not sharing one slice between the `HTTPClient` and the `websocket.DialerOptions`. It also adds a regression test.