Fix test by assigning total datasets value to stub

Since I had not defined a total datasets value for the model stub in the
test, the query of the total method was executed. Without indexes on the
ElasticSearch instance this fails. Hence, I just added this value.

With this change I also added assertions to check the formatting of the
rendered value.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Return zero score for unassertable records in RoI

I still get uncaught exeptions in the Richness of Information metric
implementation. This time, it was for a record which had no fields
defined in order to assert a Richness of Information score.

It only seemed reasonable to assign a score of 0.0 here. If there is no
information, then the score can only be the lowest possible.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Add test to specify RoI behavior on null fields

In addition to the previous commit I am now adding a test to assert the
behaviour of the Richness of Information metric in cases of documents
with null fields.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>