Normalize nullish AcroForm text values to empty strings

[Copilot]

Passing undefined into doc.formText() could serialize the literal token undefined into the field dictionary, producing an invalid PDF that breaks parsers such as pdf-lib and iText RUPS. This change normalizes explicit nullish text values to empty strings so AcroForm output remains valid.

• AcroForm value normalization

  • Treat explicit value: undefined, value: null, defaultValue: undefined, and defaultValue: null as '' during field dictionary construction.
  • Preserve existing behavior for omitted options by only normalizing keys that are actually present.

• Regression coverage

  • Add unit coverage for formText() with undefined and null values.
  • Assert those cases serialize identically to an explicit empty string.

• Example

  doc.formText('demofield', 10, 60, 200, 40, {
    value: undefined,
  });

  Before, this could emit an invalid object value like undefined. With this change, the field is serialized with an empty AcroForm value instead.

[blikblum]

@copilot

[blikblum]

Suggested change

	if (options[key] == null) {
	options[VALUE_MAP[key]] = '';
	} else {
	options[VALUE_MAP[key]] = options[key];
	}
	options[VALUE_MAP[key]] = options[key] ?? '';

simplify

[Copilot]

@copilot

Updated in 0c92a0e.