fix NPE in getBoolean when the value is json null

[JornC]

Fixes a couple of small bugs that we haven't actually run into yet.

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LLM-generated technical summary

inner.get(key) returns Java null only when the key is absent. A JSON null comes back as JSONNull.getInstance(), which is non-null, so the guard passed and isBoolean() then returned null, making booleanValue() throw an NPE. {"someFlag": null} crashes today.

The replacement is the shape getString, getNumber and getArray already use: resolve through getValue(key), narrow with isBoolean(), and null-check the narrowed value. The message also said "Number" in the boolean accessor, which is the second fix.

One behaviour change beyond the NPE: for an absent key this now throws getValue's "Did not encounter required field in object" instead of the misleading "Wrongly assumed json value to be Number". Same exception type, and consistent with the sibling accessors.

Note the module has no test files, so mvn test passes having run nothing — compilation is the only automated evidence here.

[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

[BertScholten]

LGTM (imagine it's pointless to check this in a unittest?)