(PUP-1587) crontab: make sure that invalid crontabs aren't written out

It is permissible to manage cron entries and not manage the command
property. However, if the entry doesn't exist on the target system yet,
puppet should not try to create it.

In other words, a cron resource without a should value for the command
property can only be synced to a valid state if the corresponding resource
on the target system has a valid command value already.

cron { "foo": minute => 1 }

is alright with an existing crontab of

 # Puppet Name: foo
 * * * * * /some/command

but not with a crontab not yet containing the entry named 'foo'.

This change prevents the sync operation in the error scenario.