Fix --dependency arg bug in create_adapter_plugin

[scarrucciu]

Fix bug in create_adapter_plugin when a dependency is not passed in.

resolves #2507

Description

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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[scarrucciu]

@cla-bot check

[cla-bot]

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[beckjake]

This looks great to me! I'm going to kick off the tests, though that's just a formality, and then we'll get this merged.

[beckjake]

Oh, I'm sorry - I just noticed that the changelog wasn't updated on this PR. Can you please add an entry to CHANGELOG.md under Contributors: with your github username that links to this PR? We want to make sure to credit you in the release notes!

[scarrucciu]

Oh, I'm sorry - I just noticed that the changelog wasn't updated on this PR. Can you please add an entry to CHANGELOG.md under Contributors: with your github username that links to this PR? We want to make sure to credit you in the release notes!

@beckjake can do, within the CHANGELOG.md under which version number should I add that info?

[beckjake]

@scarrucciu Please place it at the top, under dbt 0.18.0 (Release TBD). There should be an existing contributors list there, just follow that format. You can also feel free to add this to a new ### Fixes section for this release, following the existing pattern below.

[beckjake]

This is great. Thanks so much for contributing @scarrucciu! I've kicked off the tests, once they pass I'll merge.