NEXUS-7614: Fix NPE during attachment parsing

This merely fixes the symptom, but still unsure why
_attachments would contain any "non standard" fields.
Code assumed file (hence content) is always present.
NEXUS-7614: Support for stub attachments.

CoachDB have support for "stub" attachments, that does not carry
any content.
NEXUS-7614: Added UT for attachment parsing
Merge pull request #13 from sonatype/nexus-2.11.x-NEXUS-7614-npe-attachments

NEXUS-7614: Fix NPE during attachment parsing