templates: safer `record.{authors,title,abstract}`

* Changes calls to record's authors, title, abstract values to safer
  defaults.  Notably, makes sure that in the absence of a field, an
  empty string is returned instead of an exception raised.  Note: could
  be generalised even further.  (closes #119)

* Untabifies touched template files as well.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>