Crash appending empty array to WebURL.formParams

[adam-fowler]

The following code crashes

var url = WebURL("http://test.com")
let params: [(String, String)] = []
url?.formParams.append(contentsOf: params)

Line 868 is the offender

swift-url/Sources/WebURL/WebURL+FormParameters.swift

Lines 866 to 870 in 186d0b3

	var bytesWritten = 0
	if let separator = separator {
	buffer[0] = separator.codePoint
	bytesWritten += 1
	}