removing get_missing_var from ParseVar class

[jpmmcneill]

resolves #3320

Description

Gets rid of a function that only returned None that appears to be causing the compiler having issues raising descriptive exceptions when loops and vars are involved.

Getting rid of this seems to get everything working for me (ie. get more verbose errors showing up - I have tested this in jaffle_shop). I have no context as to why this odd return None function was added here so there might be an some conditional logic to make sure that the original case isn't being blown out of the water here.

Checklist

• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the CHANGELOG.md and added information about my change to the "dbt next" section.

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[jpmmcneill]

The initial naive change has lead to unit tests failing. I'll look into the reasons for failure and update below if I make progress

[jpmmcneill]

Closing this issue for the moment, I may come back to it in future.