avoid array key doesn't exist, etc PHP notices.
contributions made by admins don't have descriptions.

Avoiding:

Payment Processor Error message 9099: Error: [2052:The
'urn:APISchema.xsd:ItemName' element is invalid - The value '' is
invalid according to its datatype 'NormalizedString' - The actual length
is less than the MinLength value.]
Merge pull request #2 from jmcclelland/set-default-description

Fixed the issue 'failure when submitting donation by admin'