Array of SystemContentBloc must not be sent if text is empty

[vrcdx64]

Describe the bug
When the gateway build the payload to send to Bedrock, it can integrate a "system" array in the JSON payload which is an array of SystemContentBlock objects.

If the raw request payload sent to bedrock-access-gateway has a message object like this:

{
  "messages": [
    {
      "name": null,
      "role": "system",
      "content": ""
    },
   ... Other messages blocs...
  ]

The Bedrock request must not include it because it's length is < 1 which cause an HTTP 500 error: Invalid length for parameter system[0].text, value: 0, valid min length: 1.

Bedrock request:

{
  "modelId": "us.anthropic.claude-3-7-sonnet-20250219-v1:0",
  "messages": [
    {
      "role": "user",
      "content": [
        {
          "text": "user message.."
        }
      ]
    },
    {
      "role": "assistant",
      "content": [
        {
          "text": "assistant message..."
        }
      ]
    },
    {
      "role": "user",
      "content": [
        {
          "text": "user message..."
        }
      ]
    }
  ],
  "system": [
    {
      "text": ""
    }
  ],
  "inferenceConfig": {
    "temperature": 1.0,
    "maxTokens": 5000,
    "topP": 1.0
  }
}

Please complete the following information:

• Which API you used: /api/v1/chat/completions
• Which model you used: us.anthropic.claude-3-7-sonnet-20250219-v1:0

To Reproduce
Send a RAW payload to bedrock-access-gateway with system content set to an empty string.

Expected behavior
Bedrock request sent without the empty string to prevent HTTP 500 error.