Fix Marshalls `raw/utf8_to_base64` from erroring on empty input.

[hhhapz]

As per RFC 4648, the base64 encoding for an empty ("") input is also empty. The functionality in the Marshalls package coincedentially returns the correct result, but produces an error as well.

The variant_to_base64 function remains unchanged since an encoded variant cannot be empty.

CC: @adamscott