fix for when directory "(inst|.)/staticdocs" does not exist.

Hi Hadley, 

Behavior: 
On Windows7 32/64 bits when directory "(inst|.)/staticdocs" is non-existent, `build_package` fails with the error
'Error in if (file == "") { : argument is of length zero:'.

Reason:
When directory "(inst|.)/staticdocs" is non-existent, `pkg_sd_path(package)` returns `NULL` and subsequently  `if (file.exists....)` fails because it is a `logical(0)`.
I only checked it on W32/W64

Best regards,

Edwin