BUG: Fix out of bounds access in MetaIO

Fixed crash when value is empty or only contains white spaces.

See JIRA issue: ITK-3232 for more information

Contributed-by: Julien Finet <julien.finet@kitware.com>
Merge branch 'CrashEmptyString'

* CrashEmptyString:
  BUG: Fix out of bounds access in MetaIO