fix(LookupTable): Also skip search if field is null

[Hipska]

Base information

Question	Answer
Related to a SourceForge thread / Another PR / Combodo ticket?	N/A
Type of change?	Bug fix

Symptom

When having nullified fields as part of the lookup, you will get a lot of these warning messages:

[Warning]	No mapping found with key: '<NULL>_<NULL>', 'fieldname' will be set to zero.

Reproduction procedure

1. Create a .raw.csv file where fields used in the lookup have <NULL> as value.
2. Run synchro.

Cause

The method Lookup only skips if the fields are empty.

Proposed solution

Also take the NULL_VALUE in consideration to skip lookup.

Checklist before requesting a review

• I have performed a self-review of my code, and that it's compliant with Combodo's guidelines
• I have tested all changes I made on an iTop instance
• I have added a unit test, otherwise I have explained why I couldn't
• I have made sure the PR is clear and detailled enough so anyone can understand the real purpose without digging in the code