Fix Empty Field Handling in RIS Parser

[AliAzlanDev]

This PR enhances the parseRef function in modules/ris.js to correctly handle empty field values in RIS bibliographic files, ensuring proper citation data extraction.

Changes

• Added validation to prevent empty line values from being set as lastField
• Improved handling of fieldLookup.rl cases with empty values
• Fixed issue where subsequent field identifiers were incorrectly appended to previous field values

Example

For a RIS entry with an empty abstract field:

TY  - JOUR
PY  - 2018
AB  - 
TI  - Formulation of Herbal Gel of Antirrhinum majus Extract and Evaluation of its Anti-Propionibacterium acne Effects
ER  - 

Before: Year was incorrectly parsed as "2018\rAB - "
After: Year is correctly parsed as "2018"