Check for null type trees in dependency extraction

In some cases the dependency extraction may encounter a null `TypeTree`
(eg. arguments of macro annotations that are untyped). In such cases,
we simply ignore the node.

Fixes sbt#1593, sbt#1655.

compile/interface/src/main/scala/xsbt/Dependency.scala

@@ -139,7 +139,9 @@ final class Dependency(val global: CallbackGlobal) extends LocateClassFile {
          */
         case ident: Ident =>
           addDependency(ident.symbol)
-        case typeTree: TypeTree =>
+        // In some cases (eg. macro annotations), `typeTree.tpe` may be null.
+        // See sbt/sbt#1593 and sbt/sbt#1655.
+        case typeTree: TypeTree if typeTree.tpe != null =>
           val typeSymbolCollector = new CollectTypeTraverser({
             case tpe if !tpe.typeSymbol.isPackage => tpe.typeSymbol
           })