Optional transformed capture has incorrect type when `nil`

[natecook1000]

When a RegexBuilder pattern includes an optional, transformed capture, and the capture is nil, converting the match to AnyRegexOutput and then accessing the capture's type shows Substring? instead of the transformed type.

let pattern = Regex {
    Capture(OneOrMore(.digit)) { Int($0)! }
    Optionally {
        "."
        Capture(OneOrMore(.digit)) { Int($0)! }
    } 
}

for str in ["123.456", "789"] {
    if let match = str.wholeMatch(of: pattern) {
        print(match.output)
        let erasedMatch = Regex<AnyRegexOutput>.Match(match)
        print(erasedMatch[1].type, erasedMatch[2].type)
    }
}

// Output:
// ("123.456", 123, Optional(456))
// Int Optional<Int>
// ("789", 789, nil)
// Int Optional<Substring>

[natecook1000]

Fixed by #825.