Check for non-zero admin address when importing transparent proxy

[ericglau]

Fixes #882

When importing a proxy, the proxy kind can be inferred based on the presence of upgrade function signatures in the implementation.

For transparent proxies, we import the admin address using the admin address slot. During this import, we check whether the admin address has code.

If the proxy kind was inferred incorrectly, or if the user provided the wrong kind option (e.g. passed in 'transparent' but the proxy is actually UUPS), the admin address is the zero address. This causes the error when we check for code at the admin address:

No contract at address 0x0000000000000000000000000000000000000000 (Removed from manifest)

Instead, we should throw an error if the admin address is zero, and tell the user to set the correct proxy kind.

NOTE: This assumes that a zero admin address is never a valid scenario for a transparent proxy.

[ernestognw]

My 2 wei