add null check for stat

when using the tags plugin, this line throws sometimes an uncaught exception because `stat` is null
fix https://github.com/docpad/docpad-plugin-tags/issues/3
Merge pull request #787 from timaschew/patch-1

add null check for stat https://github.com/docpad/docpad-plugin-tags/issues/3
v6.60.4. Improvement. Bugfix.

- v6.60.4 January 27, 2014
	- Implemented `304 Not Modified` header
	- Fixed some `date` and `stat` errors (regression since v6.60.0)
		- Thanks to [Anton Wilhelm](https://github.com/timaschew) for [pull
request #787](https://github.com/bevry/docpad/pull/787)
		- Thanks to [Ivan Klimchuk](https://github.com/Alroniks) for [pull
request #781](https://github.com/bevry/docpad/pull/781)
Rebuilt meta files
Found an incorrect flag related to generations. Ref #785
v6.61.0. Bugfix.

- v6.61.0 January 27, 2014
	- Fixed regeneration always regenerating everything (regression since
v6.58.0)
Added attribution to the changelog
v6.62.0. Improvement.

- v6.62.0 January 28, 2014
	- Stylesheets are more effeciently generated
		- Files with the `outExtension: 'css'` are now the only ones included
in the `stylesheet` collection
		- Stylesheets no longer have `referencesOthers` to `true` on them by
default, this is now left up to plugin authors to do
		- Thanks to [Dimitar Kolev-Dick](https://github.com/dimitarkolev) for
[issue #789](https://github.com/bevry/docpad/issues/789)
v6.63.0. Improvement.

- v6.63.0 January 28, 2014
	- Disabled database writing by default (set `databaseCache` to `false`
instead of `'write"` by default)
		- We never used it, still many naunces with reading to figure out, so
it doesn't make sense incurring the performance penalty of the write
while there is no use for it
		- Enable writing manually by setting `databaseCache` to `"write"`
		- Enable writing and reading manually by setting `databaseCache` to
`true`
v6.63.1. Bugfix. Improvement.

- v6.63.1 January 30, 2014
	- Fixed gobbling of error messages during `contextualizeFiles`, `renderFiles` or `writeFiles` (regression since v6.59.2)
		- Thanks to [Rob Loach](https://github.com/RobLoach), [Nathan Rijksen](https://github.com/Naatan) for [issue #784](https://github.com/bevry/docpad/issues/784)
	- All task groups and tasks we define are now named (makes for easier debugging)
	- Updated dependencies
v6.63.2. Improvement.

- v6.63.2 January 30, 2014
	- Don't show generate success message if an error occured
v6.63.3. Bugfix. Improvement.

- v6.63.3 January 30, 2014
	- Regenerate every options are now customisable via `regenerateEveryOptions`
	- Regenerate every options now default to `partial:false, populate:true`
		- Before there were no defaults, meaning that they always did a non populating, partial, regeneration (regression exposed due to the fix in v6.61.0)
v6.63.4. Improvement.

- v6.63.4 February 18, 2014
	- Don't use HTTPS URLs for fetching latest version and exchange data (as HTTPS URLs may not always be available)
	- Updated dependencies
v6.63.5. Bugfix.

- v6.63.5 February 18, 2014
	- Fixed invalid TaskGroup `~3.3.7n` dependency version (regression since v6.63.1)
		- Thanks to [Jens Wilke](https://github.com/cruftex) for [issue #806](https://github.com/bevry/docpad/issues/806)
v6.63.6. Improvement.

- v6.63.6 February 18, 2014
	- Fixed npm v1.4.3 from using `^` instead of `~` when installing and updating docpad plugins