Adding data.nil? guard for deserialize

In some cases a datastream_content is nil (because lower level
exceptions are being swallowed).

This fix ensures that we don't throw a NoMethodError on
`data.force_encoding('utf-8')` when data is nil.

This manifested in the case of Curate's soft delete behavior.
Merge pull request #398 from projecthydra/fixing-rdf-datastream-encoding

Adding data.nil? guard for deserialize