Add test coverage for edge cases in empty swaps

[mantricjavier]

Underlying issue

The rejects_empty_swaps test could be expanded to cover more edge cases like:

Empty swap operations with non-empty funds
Empty funds with non-empty swap operations

#[test]
fn rejects_empty_swaps() {
    // Existing test case

    // Add test for empty swap operations with funds
    suite.execute_swap_operations(
        &creator,
        vec![],
        None,
        None,
        None,
        vec![coin(1000u128, "uwhale".to_string())],
        |result| {
            assert_eq!(
                result.unwrap_err().downcast_ref::<ContractError>(),
                Some(&ContractError::NoSwapOperationsProvided)
            )
        },
    );

    // Add test for non-empty swap operations with no funds
    suite.execute_swap_operations(
        &creator,
        swap_operations,
        None,
        None,
        None,
        vec![],
        |result| {
            assert_eq!(
                result.unwrap_err().downcast_ref::<ContractError>(),
                Some(&ContractError::NoFundsProvided)
            )
        },
    );
}

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