discard empty hooks

[drewbanin]

This might already work correctly for some adapters / hook-types. dbt should not choke on hooks that do not contain any sql.

For example:

post-hook: "{% if target.name == 'prod'%} grant usage on schema to user {% endif %}"

or

models:
  post-hook:
    # no hook here

Both of these cases (empty string / None) should be skipped in hooks