Fix #3309

When auto-completing in the REPL, we sometime tried to typecheck an
EmptyTree. Not anymore

Author: allanrenucci

compiler/src/dotty/tools/repl/ReplCompiler.scala

@@ -246,7 +246,8 @@ class ReplCompiler(val directory: AbstractFile) extends Compiler {
         case Parsed(sourceCode, trees) =>
           wrap(trees).result
         case SyntaxErrors(_, reported, trees) =>
-          if (errorsAllowed) wrap(trees).result
+          // We cannot type check an empty tree
+          if (errorsAllowed && !trees.head.isEmpty) wrap(trees).result
           else reported.errors
         case _ => List(
           new messages.Error(