Make sure input to path.{dir,base}name is a string

Since nodejs/node@08085c49b6, which will be part of Node.js v6.0,
functions from the `path` core module (like `dirname`) will require
their input to be a string.

Currently, at some points in the code they might be called
with `undefined`; This patch adds `… || ""` so that the input
is always a string.

For `path.dirname` in the babel-core file, this does not change
behaviour, since
`path.dirname(undefined) === path.dirname("") === "."` (where the
first expression is only defined for Node.js ≤ v5.x).

For `path.basename`, this changes the return value, since
`path.basename(undefined) === "undefined"` (on Node.js ≤ v5.x), but
`path.basename("") === ""`. However, it seems reasonable to assume
that, due to the trailing expression in
`path.basename(…) || "stdout"`, the current behaviour is not actually
the intended one.

There are possibly more places in the code base where similar changes
may be neccessary; However, these suffice to make the tests pass
and un-break the build of at least one external project when using
the current Node.js master branch.

packages/babel-cli/src/babel/file.js

@@ -16,7 +16,7 @@ module.exports = function (commander, filenames, opts) {
 
   let buildResult = function () {
     let map = new sourceMap.SourceMapGenerator({
-      file: path.basename(commander.outFile) || "stdout",
+      file: path.basename(commander.outFile || "") || "stdout",
       sourceRoot: opts.sourceRoot
     });