Problem

Fixes DGRAPHCORE-121
The CI build was showing sporadic failures of TestPagebufferReader2 with the error message:
Received unexpected error: EOF
The result is sporadic because the test case relied on randomized behavior. In particular, it would
generate a read-buffer of some random length, which could occasionally have a length of 0.
When the length is 0, we would encounter this error.

The cause is from the PageBufferReader having incorrect behavior for the Read(p []byte) function: In particular, when p is empty, based on the expected behavior of the Reader interface, this should return 0, nil. However, this is currently returning 0, EOF.

Solution

First:
I added a unit test to make sure we consider the empty buffer input in every case (instead of just randomly every once in a while). This doesn't actually provide a solution, but it makes sure that we don't end up with a regression in this area.
Second:
At the point where we currently detect that we read 0 bytes, before just returning 0, EOF, I add a check to make sure that the value p has non-zero length.