AVRO-2432: Handle Empty Datafiles

[kojiromike]

Fix DataFileReader so it can handle datafiles with zero blocks in them. I refactored David Beswick's patch in the ticket to close out two old TODOs in the DataFileReader iterator implementation.

Jira

• My PR addresses AVRO-2432
• My PR references the ticket in the PR title.
• My PR does not add any dependencies.

Tests

• My PR adds a test case for a datafile with an empty file data block.

Commits

• My commits all reference the Jira ticket in their subject lines.
• My commits follow the guidelines from "How to write a good git commit message

Documentation

• My PR fixes a bug in the python implementations and does not require a change to documentation.

[Fokko]

Did not know that Python would allow having a method implemented twice 😱

[kojiromike]

It's basically just a dictionary. One eats the next one. :(

>>> class foo(object):
...   def hello(self):
...     print('i am hello1')
...   def hello(self):
...     print('i am hello2')
>>> import pprint
>>> pprint.pprint(foo.__dict__)
mappingproxy({'__dict__': <attribute '__dict__' of 'foo' objects>,
              '__doc__': None,
              '__module__': '__main__',
              '__weakref__': <attribute '__weakref__' of 'foo' objects>,
              'hello': <function foo.hello at 0x7f450cd85f28>})