Normalize nullish AcroForm text values to empty strings (#1738)

* Initial plan

* Normalize nullish AcroForm text values

* Address AcroForm review feedback

* Simplify AcroForm value normalization

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

Author: Copilot

lib/mixins/acroform.js

@@ -368,8 +368,8 @@ export default {
     }
 
     Object.keys(VALUE_MAP).forEach((key) => {
-      if (options[key] !== undefined) {
-        options[VALUE_MAP[key]] = options[key];
+      if (key in options) {
+        options[VALUE_MAP[key]] = options[key] ?? '';
         delete options[key];
       }
     });

tests/unit/acroform.spec.js

@@ -186,6 +186,24 @@ describe('acroform', () => {
       expect(docData.length).toBe(3);
       expect(docData).toContainChunk(expected);
     });
+
+    test.each([
+      ['undefined', undefined],
+      ['null', null],
+    ])('nullish %s value uses an empty string', (_label, value) => {
+      const expectedDoc = new PDFDocument({
+        info: { CreationDate: new Date(Date.UTC(2018, 1, 1)) },
+      });
+      expectedDoc.initForm();
+      const expectedDocData = logData(expectedDoc);
+      expectedDoc.formText('empty', 20, 20, 50, 20, { value: '' });
+
+      doc.initForm();
+      const docData = logData(doc);
+      doc.formText('empty', 20, 20, 50, 20, { value });
+
+      expect(docData).toContainChunk(expectedDocData);
+    });
   });
 
   test('flags', () => {