[3.14] gh-148093: Raise binascii.Error from binascii.a2b_uu() on empty input (GH-149077) (GH-149350)

Instead of reading past the end of the empty buffer.
(cherry picked from commit 0c6d2f64c0c83e7652760f770ff0c5cdc5040426)

Co-authored-by: Maurycy Pawłowski-Wieroński <maurycy@maurycy.com>