Update internvl.py to fix #3528 (#3837)

Author: zodiacg

lmdeploy/vl/model/internvl.py

@@ -240,6 +240,8 @@ def proc_messages(
                 continue
             n_images = len([1 for x in message['content'] if x['type'] == 'image'])
             content = [x.get('text', '') for x in message['content'] if x['type'] == 'text']
+            if len(content) == 0:
+                content.append('')
             prompt = content[0]
             if IMAGE_TOKEN in prompt and f'<img>{IMAGE_TOKEN}' not in prompt:
                 prompt = prompt.replace(f'{IMAGE_TOKEN}', f'<img>{IMAGE_TOKEN}</img>')