test: add zero address validation tests for transfer and owner (#140)

[edehvictor]

Description

Resolves #140

This PR introduces comprehensive test coverage to ensure that zero/invalid addresses are properly rejected across the core and NFT contracts. Since Soroban does not have a native "zero address," these tests utilize a 32-byte array of zeroes to mock an invalid address execution environment.

Changes Made

• Commitment NFT (contracts/commitment_nft): - Added test_zero_address.rs.
  • Added test_nft_mint_to_zero_address_fails to verify mint panics on a zero address.
  • Added test_nft_transfer_to_zero_address_fails to verify transfer panics when to is a zero address.
• Commitment Core (contracts/commitment_core): - Added test_zero_address.rs.
  • Added test_create_commitment_zero_owner_fails to verify create_commitment panics when owner is a zero address.
• Registered the new test modules in their respective lib.rs files.

Testing Instructions

To verify these tests pass locally, run the following commands from the root directory:

cargo test --package commitment_nft
cargo test --package commitment_core