(#1717) fix for re-rendering in statement calls

[drewbanin]

Fixes #1717
Fixes #1108

This PR removes the call to render() in statement blocks. For most applications of dbt, this shouldn't change anything at all. I had to amend the run_hooks macro to render hooks, as they previously relied on statement blocks to be rendered. While I was in there, I added a check to skip queries for empty hooks, which should fix #1108.

We should think hard about if there are any other built-in parts of the dbt codebase which rely on statements re-rendering queries - I don't believe there are, but if they do exist, they're probably not explicitly tested. We should also add a note to the release notes / migration guide describing this change in behavior, as it could be very breaking for some dbt projects out there.

[beckjake]

The hooks part is pretty unfortunate, but I don't see a good way around it that doesn't break important parts of dbt, and the rest of this is really important.

Nice find! I can't believe how long this sat around causing fishy-looking behavior with nobody noticing...

[drewbanin]

I actually feel great about this hooks change. Hooks were previously kind of magic - I didn't realize that they only worked because they went through statements! I feel really good about making that particular interaction explicit, rather than implicit.

Super glad we're fixing this - just saw a similar (unrelated to hooks) issue pop up in Slack. Looks like there's a nonzero incidence of folks using code like:

{{ config(target_schema='{{ some jinja here }}') }}

which fails in terrible and confusing ways! After this change, it should fail in a single, easy to debug way haha

[beckjake]

Sorry Drew, I thought I approved this before!

[blisteringherb]

@drewbanin If we're trying to use the {{ config(target_schema='{{ env_var('SOME_ENV_VAR') }}') }} syntax, what's the proper way to use that? I'm running into errors with every permutation I can think of.

[drewbanin]

@blisteringherb that would be

{{ config(target_schema=env_var('SOME_ENV_VAR')) }}

You generally don't want to "nest" curly brackets the way you've shown in your example.

[blisteringherb]

Thank you for your help! @drewbanin