fix(dbml/core): avoid dangling 'default: ' in DbmlExporter when field.dbdefault.type is invalid

[Mrxyy]

When using dbmlExporter, if field.dbdefault.type is not set, a value of "DEFAULT " will appear. However, other Exporters will directly use field.dbdefault.type. I think it needs to be consistent.
.e.g:
MysqlExporter

OracleExporter

PostgresExporter

Summary

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Issue

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Lasting Changes (Technical)

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• (Added class EmailValidator to validate email address' validity)
• (Added Tenant#is_trial? check)

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• Documentation (if necessary)
• Tests (integration test/unit test)
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[NQPhuc]

Hi @Mrxyy,

Thank you for contributing to dbml!

To help with getting this PR merged, kindly assist in:

• Make sure the unit tests passed
• Add a test for dbdefault that cover your deafult type case. For this PR, I think you should add the test in packages/dbml-core/__test__/model_exporter/dbml_exporter/input/default_tables.in.json.