[QTI] A Text entry answer of NULL reopens empty and makes the question incomplete #6272

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Target branch: unstable

Observed behavior

• A Text entry question with the accepted answer NULL shows no inline error, but the exercise counts it as incomplete.
• Reopening the question shows that answer as an empty box with Cannot be empty.
• QTIDeclaration.coerceValue reads the string NULL as no value:

  studio/contentcuration/contentcuration/frontend/shared/views/QTIEditor/serialization/qti/QTIDeclaration.js

  Lines 212 to 216 in 954d27f

  	static coerceValue(raw, baseType) {
  	// QTI treats empty / null as NULL — see qti-is-null spec
  	if (raw === null || raw === undefined || raw === '' || raw === 'NULL') {
  	return null;
  	}

• _extractAnswers then formats it back as an empty string:

  studio/contentcuration/contentcuration/frontend/shared/views/QTIEditor/interactions/textEntry/parse.js

  Lines 108 to 114 in 954d27f

  	// for an answerless input question.
  	const values = new Set([
  	...(correctResponse ?? []).map(value => declaration.formatValue(value)),
  	...mapEntries
  	.filter(entry => entry.mappedValue >= 1)
  	.map(entry => declaration.formatValue(entry.mapKey)),
  	]);

• null and Null reopen as typed.

User-facing consequences

An author whose answer is the word NULL, as in a SQL exercise, loses it on reopen and can't complete the question with it.

Steps to reproduce

1. On unstable at 954d27f, open an exercise in the QTI editor.
2. Add a Text entry question with a prompt and the accepted answer NULL.
3. Close the editor and read the exercise's incomplete question count.
4. Reopen the question.

Acceptance Criteria

• A Text entry answer NULL, case-sensitive or not, reopens as NULL.
• validateQtiItem reports no error for a Text entry question whose only answer is NULL.
• Tests build the question with buildXML, then check both the reopened answer and validateQtiItem.

AI usage

Claude Code found this while reviewing #6150, reproduced it on unstable with a scratch Jest harness, and drafted this issue.