Fix: single blueprint edge case

[georgesittas]

This model fails in main currently:

MODEL (
  name @foo.some_table,
  kind FULL,
  blueprints ((
      foo := bar
  ))
);

SELECT
  1 AS c

With the following error:

Error: Failed to load model definition at 'playground/models/test.sql'.
Expected a key-value mapping for the blueprint value, got 'foo := bar' instead at 'playground/models/test.sql'

This PR addresses this.