fix: NullPointerException when using BoxSignRequestPrefillTag (#1121)

Fixes: #1120
Closes: SDK-2771

Author: lukaszsocha2

src/main/java/com/box/sdk/BoxSignRequestPrefillTag.java

@@ -116,7 +116,9 @@ public JsonObject getJSONObject() {
         JsonUtils.addIfNotNull(prefillTagObj, "document_tag_id", this.documentTagId);
         JsonUtils.addIfNotNull(prefillTagObj, "text_value", this.textValue);
         JsonUtils.addIfNotNull(prefillTagObj, "checkbox_value", this.checkboxValue);
-        JsonUtils.addIfNotNull(prefillTagObj, "date_value", formatAsDateOnly(this.dateValue));
+        if (this.dateValue != null) {
+            prefillTagObj.add("date_value", formatAsDateOnly(this.dateValue));
+        }
 
         return prefillTagObj;
     }

src/test/java/com/box/sdk/BoxSignRequestTest.java

@@ -12,6 +12,7 @@
 import com.github.tomakehurst.wiremock.junit.WireMockRule;
 import java.io.IOException;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.Iterator;
 import java.util.List;
 import org.junit.Before;
@@ -64,8 +65,11 @@ public void createSignRequestSucceeds() throws IOException {
         signers.add(newSigner);
 
         String parentFolderId = "55555";
+
+        BoxSignRequestCreateParams params = new BoxSignRequestCreateParams();
+        params.setPrefillTags(Collections.singletonList(new BoxSignRequestPrefillTag("id", "text")));
         BoxSignRequest.Info signRequestInfo = BoxSignRequest.createSignRequest(this.api, files,
-            signers, parentFolderId);
+            signers, parentFolderId, params);
 
         BoxFile.Info fileInfo = signRequestInfo.getSourceFiles().get(0);
         BoxSignRequestSigner signer = signRequestInfo.getSigners().get(0);