fix: Strip whitespace from SQL def before asserting starts with SELECT

[srohr]

I'm seeing a failure in alembic revision --autogenerate due to leading whitespace in definition prior to the beginning of the SQL statement, i.e.

(Pdb) definition.startswith("SELECT")
False
(Pdb) definition[0:50]
'           SELECT amazon_dsp.line_item_creative_lo'
(Pdb) definition.strip().startswith("SELECT")
True