Removing the manager destroys the asset

### Subject of the issue

If you remove the manager address, it actually behaves like you're destroying the asset.

### Your environment

algosdk 1.13.0

### Steps to reproduce

1. Use `AssetCreateTransactionBuilder` to create an asset with a manager asset.
2. Query the blockchain for the asset using `GetAssetByID` and notice it works.
3. Use `AssetConfigureTransactionBuilder` to set the manager asset to `null`.
4. Query the blockchain for the asset using `GetAssetByID` and notice it doesn't work and the asset is "destroyed".

### Expected behavior

If I had created the asset in the beginning without a manager, then it would have worked.

The expected behavior is that the `AssetConfigureTransactionBuilder` should simply remove the manager address and not destroy it.

### Actual behavior

The asset is destroyed.