Protect against empty tokens

[fm-117]

In some cases (reported in SQL scanner but to be confirmed in Cobol scanner), the scanner may be stuck in an infinte loop creating empty tokens and not advancing its index.

While this should not happen, it is safer to protect against empty tokens.

Example code reproducing the problem:

       IDENTIFICATION DIVISION.
       PROGRAM-ID. UnsupportedBySqlScanner.
       DATA DIVISION.
       WORKING-STORAGE SECTION.
       01 WS-DATE-FIN        PIC X(10).
       PROCEDURE DIVISION.
           EXEC SQL
               SELECT * FROM PROJECTS
      * KO: operator '<' is not supported by SQL scanner
               WHERE DATE_FIN < :WS-DATE-FIN
           END-EXEC
       END PROGRAM UnsupportedBySqlScanner.