Restore 2.8 behaviour which returns "localhost" for an empty url.

Fixed behaviour that caused an index out of bounds exception when merging
local metadata and causing 10s of the Maven ITs to fail. All Maven ITs
pass now using master.
diff --git a/wagon-provider-api/src/main/java/org/apache/maven/wagon/PathUtils.java b/wagon-provider-api/src/main/java/org/apache/maven/wagon/PathUtils.java
index e77e03a..4641473 100644
--- a/wagon-provider-api/src/main/java/org/apache/maven/wagon/PathUtils.java
+++ b/wagon-provider-api/src/main/java/org/apache/maven/wagon/PathUtils.java
@@ -133,6 +133,10 @@
      */
     public static String host( final String url )
     {
+        if ( url == null || url.length() == 0 )
+        {
+            return "localhost";
+        }
         String authorization = authorization( url );
         int index = authorization.indexOf( '@' );
         String host = ( index >= 0 ) ? authorization.substring( index + 1 ) : authorization;