[Annotation] Use the field V entry when there is no Parent one for a radio button (bug 1860602)

src/core/annotation.js

@@ -3196,7 +3196,7 @@ class ButtonWidgetAnnotation extends WidgetAnnotation {
   }
 
   _processRadioButton(params) {
-    this.data.fieldValue = this.data.buttonValue = null;
+    this.data.buttonValue = null;
 
     // The parent field's `V` entry holds a `Name` object with the appearance
     // state of whichever child field is currently in the "on" state.

test/pdfs/bug1860602.pdf.link

@@ -0,0 +1,2 @@
+https://bugzilla.mozilla.org/attachment.cgi?id=9359849
+

test/test_manifest.json

@@ -8170,5 +8170,16 @@
       "rounds": 1,
       "type": "eq",
       "annotations": true
+   },
+   {
+      "id": "bug1860602",
+      "file": "pdfs/bug1860602.pdf",
+      "md5": "37de2fbd0a02c5a3ca27c6e75fea8932",
+      "rounds": 1,
+      "link": true,
+      "firstPage": 2,
+      "lastPage": 2,
+      "type": "eq",
+      "forms": true
    }
 ]