Fix bug in tortoise.models.Model When a QuerySet uses the only function and then uses the print function to print the returned result, an AttributeError is generated

[gck123]

…tion and then uses the print function to print the returned result, an AttributeError is generated

Description

old:

def _get_pk_val(self) -> Any:
        return getattr(self, self._meta.pk_attr)

new:

def _get_pk_val(self) -> Any:
        return getattr(self, self._meta.pk_attr, None)

Motivation and Context

When a QuerySet uses the only function and then uses the print function to print the returned result, an AttributeError is generated
https://github.com/tortoise/tortoise-orm/issues/1723

How Has This Been Tested?

make ci

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[coveralls]

Pull Request Test Coverage Report for Build 11091622517

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 89.011%

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Totals
Change from base Build 11067278859:	0.0%
Covered Lines:	5965
Relevant Lines:	6586

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💛 - Coveralls