Blockchain.stream()/Block() fail with only_ops=True on blocks without ops

[crokkon]

Expected behavior

blockchain.stream(start, stop, only_ops=True) should return all operations, including virtual ops, contained in the block range from start to stop. The same should be possible via Block(blocknum, only_ops=True) for individual blocks. If a block does not contain operations, the calls should return an empty list.

Actual behavior

blockchain.stream(start, stop, only_ops=True) and Block(blocknum, only_ops=True) fail with a BlockDoesNotExistsException if the block contains no operations.

How to reproduce

from beem.blockchain import Blockchain
b = Blockchain()
for op in b.stream(start=1000, stop=1001, only_ops=True):
    print(op)

or

from beem.block import Block
b = Block(1000, only_ops=True)

Output:

Traceback (most recent call last):
  File "bug_get_ops_in_block.py", line 8, in <module>
    b = Block(1000, only_ops=True)
  File "/usr/local/lib/python3.6/site-packages/beem/block.py", line 76, in __init__
    steem_instance=steem_instance
  File "/usr/local/lib/python3.6/site-packages/beem/blockchainobject.py", line 134, in __init__
    self.refresh()
  File "/usr/local/lib/python3.6/site-packages/beem/block.py", line 156, in refresh
    raise BlockDoesNotExistsException("output: %s of identifier %s" % (str(block), str(self.identifier)))
beem.exceptions.BlockDoesNotExistsException: output: {} of identifier 1000

Environment

# beempy --version
beempy, version 0.20.17
# python --version
Python 3.6.6