blob object constructor should use the static null_blob when blob length = 0
Currently the blob constructor creates a BLOB whose priv pointer is NULL when the encoding is the empty string:
new empty = blobcode.blob(encoded="");
An empty BLOB is a legitimate object, but priv==NULL does not play well with other VMODs expecting a BLOB.
null_blob, whose priv pointer points to the static constant empty string, was created for this purpose, and is returned by the
decode function when the encoding is the empty string. The constructor should use it as well.