Marshall Bruner 1402508416
give better error if required field of url join is invalid (#10589)
# Description
Fix #10506 by adding `ExpectedNonNull` `ShellError` if required field is
entered as `$nothing`, `null`, " ", etc.

This adds a new `ShellError`, `ExpectedNonNull`, taking the expected
type and span.

# User-Facing Changes
Will get a more helpful error in the case described by #10506. Examples:
```nushell
➜ {scheme: "", host: "github.com"} | url join
Error: nu:🐚:expected_non_null

  × Expected string found null.
   ╭─[entry #16:1:1]
 1 │ {scheme: "", host: "github.com"} | url join
   ·          ─┬
   ·           ╰── expected string, found null
   ╰────
```

```nushell
❯ {scheme: "https", host: null} | url join
Error: nu:🐚:expected_non_null

  × Expected string found null.
   ╭─[entry #19:1:1]
 1 │ {scheme: "https", host: null} | url join
   ·                         ──┬─
   ·                           ╰── expected string, found null
   ╰────
```

# Tests + Formatting
All pass.
2023-10-10 19:24:23 +02:00
..
2023-10-10 03:31:15 +13:00
2023-10-10 03:31:15 +13:00
2023-10-10 03:31:15 +13:00
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