use `default = null` instead of `default = ""` in variables where possible

[phorcys420]

a lot of our modules have default = "" variables instead of default = null, i think this is a bad practice and makes code less understandable.

[matifali]

@DevelopmentCats and I recently discussed this and even considered adding a lint rule to prevent it.

[phorcys420]

good idea, we can make the lint and then proceed to fix it, we will have to be careful to edit the associated ternary statements though (there's var.something != "" ? var.something : "" everywhere)

---

we should also consider switching to try(var.something, "") instead, it is cleaner.
(there is also coalesce, but we can't use it for a fallback to an empty string because coalesce takes any number of arguments and returns the first one that isn't null or an empty string.)

for example:

  url = join("", [
    var.protocol,
    "://coder.coder-remote/open",
    "?owner=",
    data.coder_workspace_owner.me.name,
    "&workspace=",
    data.coder_workspace.me.name,
-   var.folder != "" ? join("", ["&folder=", var.folder]) : "",
+   # this would be fine, because passing "&folder=" if the folder variable is empty or null then that works
+   join("", ["&folder=", try(var.folder, "")]),
    var.open_recent ? "&openRecent" : "",
    "&url=",
    data.coder_workspace.me.access_url,
    "&token=$SESSION_TOKEN",
  ])

there are also other ways we could just handle null cases better since it is null and not an empty string.
some good examples here: https://www.codeterraform.com/post/successfully-deal-with-terraforms-null-value