Check for post_provisioning_scripts being defined before attempting t…

[mminklet]

…o run them

Was getting this error before this check
file or module does not exist: /Users/Mike/drupalvm/provisioning/post_provision_scripts

[geerlingguy]

If you have post_provision_scripts: [] inside config.yml, this task doesn't fail, it just skips due to the list being empty (see: https://github.com/geerlingguy/drupal-vm/blob/master/example.config.yml#L215-L218)

In this case, if upgrading from an older version of Drupal VM, you should run a diff of your older config.yml with the example.config.yml to make sure any new vars are added in. It's not ideal, and I might find a better way to deal with this problem (adding new variables) soon, but it is what it is for now and if I add a conditional for one new variable I'd have to do it for all of them :(

[geerlingguy]

I'll keep this open for now, per discussion in #425 — as "MINOR version when you add functionality in a backwards-compatible manner, and" would mean a 2.X.x release shouldn't break 2.X-1.x code/config... :(