Never flush without items

[gzuidhof]

In a shutdown situation it was possible flush was called with an empty items slice - this prevents that and adds a test case for it.

This would happen when:

• The context has been cancelled
• The buffer is empty
• The current items array is empty.

skipFlush would be false:

isCancelled && len(b.buffer) > 0 && !maxSize

the len(b.buffer) > 0 would be false, but there was no check whether there are any entries to be flushed.

[mikemherron]

👍