Avoid crash if form is nil when filling

pdf/model/form.go

@@ -246,6 +246,9 @@ type FieldValueProvider interface {
 
 // Fill populates `form` with values provided by `provider`.
 func (form *PdfAcroForm) Fill(provider FieldValueProvider) error {
+	if form == nil {
+		return nil
+	}
 	objMap, err := provider.FieldValues()
 	if err != nil {
 		return err

pdf/model/form_test.go

@@ -155,3 +155,8 @@ endobj
 		t.Fatalf("Mismatch in expected and actual field dictionaries (deep)")
 	}
 }
+
+func TestFormNil(t *testing.T) {
+	var form *PdfAcroForm
+	form.Fill(nil)
+}