Make .ext on empty paths return "" rather than crashing with a NPE (#87)

Fixes com-lihaoyi/Ammonite#1017

This is more consistent with calling `.ext` on non-empty paths which have no extension, since those currently return `""`

We also introduce a new a `lastOption`  method, as a way to get the last segment in a list without throwing. Not sure if that's the right thing to do, or whether we should make `.last` return `""` (which is otherwise not a valid return value, since path segments cannot be empty).

Covered with simple unit tests

os/src-jvm/ResourcePath.scala

@@ -23,7 +23,7 @@ class ResourcePath private[os](val resRoot: ResourceRoot, segments0: Array[Strin
   def toSource = new Source.WritableSource(getInputStream)
   val segments: IndexedSeq[String] = segments0
   type ThisType = ResourcePath
-  def last = segments0.last
+  def lastOpt = segments0.lastOption
   override def toString = resRoot.errorName + "/" + segments0.mkString("/")
   protected[this] def make(p: Seq[String], ups: Int) = {
     if (ups > 0){

os/src/Path.scala

@@ -182,9 +182,14 @@ trait BasePathImpl extends BasePath{
   def /(chunk: PathChunk): ThisType
 
   def ext = {
-    val li = last.lastIndexOf('.')
-    if (li == -1) ""
-    else last.slice(li+1, last.length)
+    lastOpt match{
+      case None => ""
+      case Some(lastSegment) =>
+        val li = lastSegment.lastIndexOf('.')
+        if (li == -1) ""
+        else last.slice(li+1, last.length)
+    }
+
   }
 
   override def baseName: String = {
@@ -193,7 +198,9 @@ trait BasePathImpl extends BasePath{
     else last.slice(0, li)
   }
 
-  def last: String
+  def last: String = lastOpt.getOrElse(throw PathError.LastOnEmptyPath())
+
+  def lastOpt: Option[String]
 }
 
 object PathError{
@@ -208,6 +215,9 @@ object PathError{
 
   case class NoRelativePath(src: RelPath, base: RelPath)
     extends IAE(s"Can't relativize relative paths $src from $base")
+
+  case class LastOnEmptyPath()
+    extends IAE("empty path has no last segment")
 }
 
 /**
@@ -239,7 +249,7 @@ object FilePath {
   */
 class RelPath private[os](segments0: Array[String], val ups: Int)
   extends FilePath with BasePathImpl with SegmentedPath {
-  def last = segments.last
+  def lastOpt = segments.lastOption
   val segments: IndexedSeq[String] = segments0
   type ThisType = RelPath
   require(ups >= 0)
@@ -305,7 +315,7 @@ object RelPath {
   */
 class SubPath private[os](val segments0: Array[String])
   extends FilePath with BasePathImpl with SegmentedPath {
-  def last = segments.last
+  def lastOpt = segments.lastOption
   val segments: IndexedSeq[String] = segments0
   type ThisType = SubPath
   protected[this] def make(p: Seq[String], ups: Int) = {
@@ -433,7 +443,7 @@ class Path private[os](val wrapped: java.nio.file.Path)
   def segmentCount = wrapped.getNameCount
   type ThisType = Path
 
-  def last = wrapped.getFileName.toString
+  def lastOpt = Option(wrapped.getFileName).map(_.toString)
 
   def /(chunk: PathChunk): ThisType = {
     if (chunk.ups > wrapped.getNameCount) throw PathError.AbsolutePathOutsideRoot

os/test/src/PathTests.scala

@@ -47,6 +47,24 @@ object PathTests extends TestSuite{
           assert((base / "baseOnly").ext == "")
           assert((base / "baseOnly.").ext == "")
         }
+
+        test("emptyExt"){
+          os.root.ext ==> ""
+          os.rel.ext ==> ""
+          os.sub.ext ==> ""
+          os.up.ext ==> ""
+        }
+
+        test("emptyLast"){
+          intercept[PathError.LastOnEmptyPath](os.root.last).getMessage ==>
+            "empty path has no last segment"
+          intercept[PathError.LastOnEmptyPath](os.rel.last).getMessage ==>
+            "empty path has no last segment"
+          intercept[PathError.LastOnEmptyPath](os.sub.last).getMessage ==>
+            "empty path has no last segment"
+          intercept[PathError.LastOnEmptyPath](os.up.last).getMessage ==>
+            "empty path has no last segment"
+        }
       }
 
       test("RelPath"){