[pysqlite] Blob.read() return value

The documentation for Blob.read() indicates that the function will
only ever return strings or bytes, but it looks like it return Nones
and NULL (does that become None?) under certain circumstances. What
exactly are the conditions under which read() will return something
other than a string or bytes? And could that be put in the
documentation? This bit me a bit today from a call embedded in a c++
program, and I'll admit that debugging it was a bit of a learning
experience!

Austin