feat: add 'isNotEmpty' assertion operator

[sanjai0py]

Description

This PR adds support for the isNotEmpty assertion operator. The isNotEmpty assertion checks that a value is not null, not 0, not false, not an empty string, not an empty array, and not an empty object. The implementation includes:

• Checking that the value is not one of [null, 0, false, '', []].
• Ensuring arrays have a length greater than 0.
• Ensuring objects have at least one key.

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Contribution Checklist:

• The pull request only addresses one issue or adds one feature.
• The pull request does not introduce any breaking changes
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