Fix failure for empty string match

Trying to match the empty string on descendants (or self) throws an
unexpected error:

    scala> <x/> \\ ""
    java.lang.StringIndexOutOfBoundsException: String index out of range: 0
      at java.lang.String.charAt(String.java:646)
      at scala.collection.immutable.StringOps$.apply$extension(StringOps.scala:37)
      at scala.xml.NodeSeq.$bslash$bslash(NodeSeq.scala:147)
      ...

The error should be consistent with what is thrown when trying to
match the empty string on just children:

    scala> <x/> \ ""
    java.lang.IllegalArgumentException
      at scala.xml.NodeSeq.fail$1(NodeSeq.scala:97)
      at scala.xml.NodeSeq.$bslash(NodeSeq.scala:120)
      ...

This was identified while writing ScalaCheck property tests.

Author: ashawley

shared/src/main/scala/scala/xml/NodeSeq.scala

@@ -140,8 +140,10 @@ abstract class NodeSeq extends AbstractSeq[Node] with immutable.Seq[Node] with S
    *  The document order is preserved.
    */
   def \\(that: String): NodeSeq = {
+    def fail = throw new IllegalArgumentException(that)
     def filt(cond: (Node) => Boolean) = this flatMap (_.descendant_or_self) filter cond
     that match {
+      case ""                  => fail
       case "_"                 => filt(!_.isAtom)
       case _ if that(0) == '@' => filt(!_.isAtom) flatMap (_ \ that)
       case _                   => filt(x => !x.isAtom && x.label == that)

shared/src/test/scala/scala/xml/XMLTest.scala

@@ -145,6 +145,16 @@ class XMLTest {
     assertEquals(expected, actual)
   }
 
+  @UnitTest(expected=classOf[IllegalArgumentException])
+  def failEmptyStringChildren: Unit = {
+    <x/> \ ""
+  }
+
+  @UnitTest(expected=classOf[IllegalArgumentException])
+  def failEmptyStringDescendants: Unit = {
+    <x/> \\ ""
+  }
+
   @UnitTest
   def namespaces: Unit = {
     val cuckoo = <cuckoo xmlns="http://cuckoo.com">