DecodeBase64 returns empty string on *all* errors

[SeanCurtis-TRI]

It already returns an empty string if it encounters an invalid character. However, if, instead, it has an incomplete set of characters (no-multiple of four), it currently returns a truncated result. Instead, it detects truncation and returns the empty string.

The only time DecodeBase64() returns a valid empty return value is if the input was likewise empty.