Additional null checks and comments

Add defensive coding to check for possible null pointer values, thus
preventing certain potential runtime exceptions.
Fix some minor spelling inconsistencies.
Refactor some method signatures and update the Javadoc.
Reformat Resource toString() method to avoid lengthy String
concatenation and duplicated separators.

Change-Id: I4b01eb844700e00d0c909bcc5fa2dbc91d5149e4
Issue-ID: AAI-2281
Signed-off-by: mark.j.leonard <mark.j.leonard@gmail.com>