Fix excel table area detection failures

[ccleva]

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Description

Blank cells detection (used to identify the table area in excel sheets) did not take into account formulas and wrongly identified numerical 0 or false boolean as blank, leading to failed or incorrect table reading.

Fixes #1307

Testing

Added tests to confirm the issues. Tests are now passing with the fix.

[sonarqubecloud]

Quality Gate passed

Issues
4 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[ccleva]

Sonar complains about test methods being public, let me know if you prefer:

• to keep the new methods public for consistency with the other test methods
• to change the new test methods to package as recommended by Sonar
• to change all test methods to package for consistency