Fix regression XML parsing null with keepStrings

[marilynel]

Fixes #986

Made keepStrings account for null in XML; fixing regression from previous commit.
Refactored two XML tests to include null.
Unit tests ran with gradlew.

[sonarqubecloud]

Quality Gate passed

Issues
0 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

[stleary]

What problem does this code solve?
Fixes a regression in XML parsing where null strings are not handled correctly when keepStrings is set

Does the code still compile with Java6?
Yes

Risks
Low

Changes to the API?
No

Will this require a new release?
No

Should the documentation be updated?
No

Does it break the unit tests?
No, new unit tests were added

Was any code refactored in this commit?
XML null value testing was updated

Review status
APPROVED

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