LibURL: Allow inputs containing only whitespace

The check for:

```
    if (start_index >= end_index)
        return {};
```

To prevent an out of bounds when trimming the start and end of the input
of whitespace was preventing valid URLs (only having whitespace in the
input) from being parsed.

Instead, prevent start_index from ever getting above end_index in the
first place, and don't treat empty inputs as an error.

Fixes one WPT test on:

https://wpt.live/url/url-constructor.any.html

Tests/LibWeb/Text/expected/URL/url.txt

@@ -108,6 +108,16 @@ port => '9000'
 pathname => '/path'
 search => '?query'
 hash => '#frag'
+new URL('  \t', 'http://ladybird.org/foo/bar')
+protocol => 'http:'
+username => ''
+password => ''
+host => 'ladybird.org'
+hostname => 'ladybird.org'
+port => ''
+pathname => '/foo/bar'
+search => ''
+hash => ''
 =========================================
 URL.parse('ftp://serenityos.org:21', undefined)
 protocol => 'ftp:'
@@ -219,3 +229,13 @@ port => '9000'
 pathname => '/path'
 search => '?query'
 hash => '#frag'
+URL.parse('  \t', 'http://ladybird.org/foo/bar')
+protocol => 'http:'
+username => ''
+password => ''
+host => 'ladybird.org'
+hostname => 'ladybird.org'
+port => ''
+pathname => '/foo/bar'
+search => ''
+hash => ''