Don’t add `file` property if no file was given

Previously, if you didn’t pass the `file` option to
`new SourceMapGenerator()` or
`SourceNode.prototype.toStringWithSourceMap()`, `"file": null` would be
present in the source map. Now, the file property isn’t added at all
instead, which is more intuitive.
Merge pull request #104 from lydell/no-file-no-null

Don’t add `file` property to the generated source map if no file was given.