fix: raw_call type when max_outsize=0 is set

prior to this commit, when `raw_call` is used with `max_outsize`
explicitly set to 0 (`max_outsize=0`) the compiler incorrectly infers that
raw_call has no return type

```vyper
@external
@payable
def foo(_target: address):
    # compiles
    a: bool = raw_call(_target, method_id("foo()"), revert_on_failure=False)
    # should have same behavior, but prior to this commit does not compile:
    b: bool = raw_call(_target, method_id("foo()"), max_outsize=0, revert_on_failure=False)
```

chainsec june 2023 review 5.16

vyper/builtins/functions.py

@@ -1093,7 +1093,7 @@ def fetch_call_return(self, node):
         revert_on_failure = kwargz.get("revert_on_failure")
         revert_on_failure = revert_on_failure.value if revert_on_failure is not None else True
 
-        if outsize is None:
+        if outsize is None or outsize.value == 0:
             if revert_on_failure:
                 return None
             return BoolT()