Bug: Optional[Union] does not bind values as would be expected

```
MyValueType = u64 | str

def func1(value: Optional[MyValueType] = None):
if value is None:
print("No value provided")
else:
print(f"Value provided: {value}")

func1("a")
```

Prints compilation error:
`optional_test.codon:15 (1-11): error: 'Optional[str]' does not match expected type 'Optional[Union[UInt[64] | str]]'`

Line 15 is the last line `func1("a")`