Support new agate Integer type and test with empty seed

[gshank]

resolves #935

Problem

dbt Core added support for an agate integer type to support the show command. Empty seeds were failing due to lack of support in the "convert_" methods in impl.py, so a regression bug case fixed that.

Solution

Add a "convert_integer_type" method and include test of empty seed creation.

Checklist

• [x] I have read the contributing guide and understand what's expected of me
• [x] I have run this code in development and it appears to resolve the stated issue
• [x] This PR includes tests, or tests are not required/relevant for this PR
• [ ] This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX

[github-actions[bot]]

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