add todo, and update docs

Author: dlarocque

docs-devsite/ai.templategenerativemodel.md

@@ -85,8 +85,7 @@ Makes a single non-streaming call to the model and returns an object containing
 <b>Signature:</b>
 
 ```typescript
-generateContent(templateId: string, templateVariables: object, // anything!
-    singleRequestOptions?: SingleRequestOptions): Promise<GenerateContentResult>;
+generateContent(templateId: string, templateVariables: object, singleRequestOptions?: SingleRequestOptions): Promise<GenerateContentResult>;
 ```
 
 #### Parameters

packages/ai/src/methods/chat-session.ts

@@ -187,6 +187,7 @@ export class ChatSession {
         // errors propagate to the user's `result.response` promise.
         if (response.candidates && response.candidates.length > 0) {
           this._history.push(newContent);
+          // TODO: Validate that `response.candidates[0].content` is not null.
           const responseContent = { ...response.candidates[0].content };
           if (!responseContent.role) {
             responseContent.role = 'model';