author without affiliation extraction error

[krzysztof]

This line can fail if 'author' is in 'r' but affiliation list is empty:
https://github.com/inveniosoftware/invenio-files-processor/blob/master/invenio_files_processor/processors/pdf_grobid.py#L66

Traceback (most recent call last):
  File "/home/krnowak/src/invenio-files-processor/invenio_files_processor/views.py", line 55, in extract_pdf_metadata
    metadata = processor.process(object_version)
  File "/home/krnowak/src/invenio-files-processor/invenio_files_processor/processors/pdf_grobid.py", line 67, in process
    for it in r['authors']] if 'authors' in r else None
  File "/home/krnowak/src/invenio-files-processor/invenio_files_processor/processors/pdf_grobid.py", line 67, in <listcomp>
    for it in r['authors']] if 'authors' in r else None
IndexError: list index out of range
127.0.0.1 - - [09/Aug/2017 11:36:03] "POST /filesprocessor/pdf_grobid/95bb3c60-8727-4dde-8b2f-863a53d1414a HTTP/1.1" 500 -