ForeignHash does not consider 'default_value' of original Hash

[Endogen]

Describe the bug
Defining a ForeignHash in a contract doesn't take in account the default_value of the original Hash.

To Reproduce
Steps to reproduce the behavior:

1. Create a new contract and define a ForeignHash. For example:

tau_balances = ForeignHash(foreign_contract='currency', foreign_name='balances')

2. Retrieve a value that does NOT exist:

some_value = tau_balances["this does not exist"]

Expected behavior
Variable some_value should be 0 since the original Hash was defined with default_value=0 but it's None