[Bug] nil pointer panic when calling role related functions with no role definition model

[MuZhou233]

Describe the bug
Currently role_definition is an optional section in the model definitions and the role assertions (model["g"]) won't be initialized if it is omited. However, the other parts of the code won't check if it is initialized before use and result in an nil pointer panic (e.g. some lines directly read model["g"]["g"] or even more without check). This affects a wide range of public and private functions.

To Reproduce
Steps to reproduce the behavior:

1. Initialize the model with no role_definition section
2. Call a function that related to role
3. Some of them may return, most of them will panic

Expected behavior
The panic should not be happen. Involve nil checks to return an error rather than panic.

Additional context
According to the documentation, this is a misuse. Following the guidelines would not encounter this issue.

[casbin-bot]

@tangyang9464 @JalinWang

[github-actions]

🎉 This issue has been resolved in version 2.89.0 🎉

The release is available on GitHub release

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