Add unit tests for sell revert when creator has zero total supply

[Chucks1093]

Summary
Attempting to sell keys when the creator has zero total supply is an impossible state, but the error handling for this edge is not explicitly tested. A unit test confirms the contract reverts with the expected error rather than producing an arithmetic panic.

Scope

• add a unit test that calls the sell function against a creator with zero total supply
• assert the call reverts with the expected error variant
• assert no state changes occur as a result of the failed call

Acceptance Criteria

• a sell against a zero-supply creator reverts with the correct error
• no state mutation occurs from the failed call
• the test is distinct from the insufficient balance sell revert test

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[jaynomyaro]

@jaynomyaro has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi! I would love to work on this issue, Could you please assign this to me,and I promise to have it resolved ASAP,if is still available. Thanks!

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[HybridDan]

@HybridDan has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi,As a senior developer I'll love to work on this issue. Please Assign me.

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[khanavi272-spec]

@khanavi272-spec has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

I would like to work on this issue

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