Fixes #1375 - Check for empty filenames.

Starting sbt in root isn't a great idea, but it shouldn't break.

main/src/main/scala/sbt/Project.scala

@@ -242,7 +242,10 @@ object Project extends ProjectExtra {
   def normalizeProjectID(id: String): Either[String, String] =
     {
       val attempt = normalizeBase(id)
-      val refined = if (!validProjectIDStart(attempt.substring(0, 1))) "root-" + attempt else attempt
+      val refined =
+        if (attempt.length < 1) "root"
+        else if (!validProjectIDStart(attempt.substring(0, 1))) "root-" + attempt
+        else attempt
       validProjectID(refined).toLeft(refined)
     }
   private[this] def normalizeBase(s: String) = s.toLowerCase(Locale.ENGLISH).replaceAll("""\W+""", "-")

main/src/test/scala/ProjectSpec.scala

@@ -0,0 +1,18 @@
+package sbt
+
+import org.specs2.Specification
+
+class ProjectSpec extends Specification {
+  def is = s2"""
+
+  This is a specification to check utility methods on the Project object
+
+  Project should
+    normalize projectIDs if they are empty  ${normalizeEmptyFileName}
+
+  """
+
+  def emptyFilename = ""
+
+  def normalizeEmptyFileName = Project.normalizeProjectID(emptyFilename) must equalTo(Right("root"))
+}