ValueError: invalid literal for int() with base 16: '0x'
### Environment information

* `brownie` Version: master
* `ganache-cli` Version:  v6.12.2 (ganache-core: 2.13.2)
* `solc` Version: N/A
* Python Version: 3.8.6
* OS: osx and linux

### What was wrong?
I was helping a friend set up the env in windows and got the following error from the console
```
Brownie environment is ready.
>>> weth = Contract("0xC02aaA39b223FE8D0A0e5C4F27eAD9083C756Cc2")
  File "<console>", line 1, in <module>
  File "eth_brownie-1.13.0-py3.8.egg/brownie/network/contract.py", line 923, in __init__
    contract = self.from_explorer(address, owner=owner, silent=True)
  File "eth_brownie-1.13.0-py3.8.egg/brownie/network/contract.py", line 1102, in from_explorer
    if int(implementation_eip1967.hex(), 16):
ValueError: invalid literal for int() with base 16: '0x'
```

### How can it be fixed?
I will send a PR :)

diff --git a/brownie/network/contract.py b/brownie/network/contract.py
index 6a90bbfe..174f6b1e 100644
--- a/brownie/network/contract.py
+++ b/brownie/network/contract.py
@@ -1099,9 +1099,9 @@ class Contract(_DeployedContractBase):
             )
             # always check for an EIP1822 proxy - https://eips.ethereum.org/EIPS/eip-1822
             implementation_eip1822 = web3.eth.getStorageAt(address, web3.keccak(text="PROXIABLE"))
-            if int(implementation_eip1967.hex(), 16):
+            if len(implementation_eip1967) > 0 and int(implementation_eip1967.hex(), 16):
                 as_proxy_for = _resolve_address(implementation_eip1967[12:])
-            elif int(implementation_eip1822.hex(), 16):
+            elif len(implementation_eip1822) > 0 and int(implementation_eip1822.hex(), 16):
                 as_proxy_for = _resolve_address(implementation_eip1822[12:])
             elif data["result"][0].get("Implementation"):
                 # for other proxy patterns, we only check if etherscan indicates
['brownie/network/contract.py']