avoid nil package resource name for pkg_php_mbstring

As reported in issue #2, the nil value in `pkg_php_mbstring` makes postfixadmin-cookbook's default recipe fail for me with an error message that package resource names must not be nil.

This patch always assigns a non-nil name to the package resource and specifies the package name via the `package_name` resource attribute. This way the `not_if` guard becomes effective.

Regards
Christoph