Make sure that IDNA's toASCII operation does not return an empty string.

Addresses whatwg/url#497

CHANGELOG.md

@@ -6,6 +6,7 @@
 
 - `URLSearchParams::sort()` now correctly sorts by code units instead of trying to fake it using code points.
 - Null bytes are now percent encoded in fragments instead of being ignored per [whatwg/url#440](https://github.com/whatwg/url/issues/440).
+- The host of URLs with special schemes can no longer be an empty string per [whatwg/url#497](https://github.com/whatwg/url/pull/497).
 
 ### Fixed
 

src/Component/Host/HostParser.php

@@ -37,7 +37,14 @@ private function domainToAscii(string $domain, bool $beStrict = false): StringHo
         }
 
         try {
-            return new StringHost(IDNA::toAscii($domain, $flags));
+            $result = IDNA::toAscii($domain, $flags);
+
+            if ($result === '') {
+                // Validation error.
+                throw new IDNATransformException();
+            }
+
+            return new StringHost($result);
         } catch (IDNATransformException $e) {
             // Validation error.
             throw $e;