Fix null-pointer dereferences flagged by CodeQL java/dereferenced-value-may-be-null

[vharseko]

Summary

Resolves 16 real java/dereferenced-value-may-be-null CodeQL alerts by fixing
genuine null-pointer dereferences. Each is a place where a value that can be null
— per an explicit guard elsewhere, or an unassigned local — is dereferenced.

Fixes

Module	File	Defect
rest/api-descriptor	AsciiDocTable	Range check unboxed this.columnsPerRow (provably null there) instead of the int parameter — the setter always threw NPE.
json-web-token	JWK	The new JsonException(...) guarding a null kty was never thrown, so kty reached kty.toString() (NPE). Added the missing throw (kty is required per RFC 7517 §4.1). kid is optional (RFC 7517 §4.5), so its dead exception is dropped and it is only stored when present.
audit/core	DateUtil	Integer result, returned as int, was null for null input dates → unboxing NPE. Now throws IllegalArgumentException.
script/common	ScriptRegistryImpl	Guard used || instead of &&, so source/dependency could be null when dereferenced.
script/common	ScriptExecutor	Empty if (null == scriptEngine) body fell through to .eval(). Now throws ScriptException.
persistit/core	ManagementImpl	finally dereferenced a possibly-unassigned exchange, masking the real exception. Guarded with a null check.
persistit/core	DefaultObjectCoder	Two catch handlers called accessor.toString() with accessor possibly null. Use String.valueOf(accessor).
persistit/ui	AdminUI	newPanel.setDefaultButton() outside its null guard; _buttonList.size() after a null-tolerating block.
persistit/ui	AdminUITreePanel	treeInfoArray.length without a null check.
persistit/core	Exchange	Traversal loop dereferenced buffer without the null guard applied just above; added && buffer != null to the loop condition.
persistit/core	RecoveryManager	lastRequiredPageNode (set in lockstep with the gating address) dereferenced; guarded the validate() call to make the invariant explicit.
persistit/core	BufferPool	invalidateSmallVolume (per-page) requires a concrete volume; the copy-pasted || volume == null match clauses were dead code that made the analyzer treat volume as nullable. Removed them — behaviour-identical for a real volume; the live "invalidate all" idiom stays in invalidateLargeVolume.
persistit/ui	InspectorPanel	Fetcher re-read the record field; pass the already null-checked local instead — removes the potential NPE and a TOCTOU race.

Behaviour

Non-error paths are unchanged; the changes only avoid NPEs that were either
guaranteed (AsciiDocTable, JWK) or latent on error/edge paths. JWK keeps
kid optional (RFC 7517 §4.5) so the keyless JWKs built from the JWE spec
examples continue to parse.

Not in this PR

Of the 25 alerts for this rule, the other 9 are high-confidence false positives
(correlated invariants CodeQL cannot prove — e.g. boxed primitives in
ValueInspectorTreeNode, a corrupt() that always throws in Exchange) and were
dismissed on GitHub.

Testing

mvn -o -pl commons/rest/api-descriptor,commons/json-web-token,commons/audit/core,script/common,persistit/core,persistit/ui compile — all six modules compile.
mvn -o -pl commons/json-web-token test — 328 tests, 0 failures, 0 errors.