Disallow null values in test mode data

[arunmahadevan]

Right now if 'null' value is present, the test run fails but does not throw any useful error message. Apparently it fails in Collectors.toMap().

Since we remove the fields with null value in AvroStreamsSnapshotDeserializer we should also skip null values in the test case data.

[priyank5485]

@arunmahadevan For HDF-3.0-maint branch we decided to put the fix where we dont put the fields with null values. However, because we could not reach a solution, the master still has the issue where if data has null fields it will throw an error in worker. Just pointing it out so that once we fix the issue of handling null fields, we can go back and add data with null fields for test mode. For now, we can remove null fields.

[arunmahadevan]

Yes for now we can disable null values.