fix: fixed bug in collection of text context parts

[adubovik]

Analytics didn't correctly collect content from the text content parts like this one:

{
  "messages": [
    {
      "role": "system",
      "content": [
        {
          "type": "text",
          "text": "text!"
        }
      ]
    },
    { "role": "user", "content": "test!" }
  ]
}

because of a typo in the content field name:

ai-dial-analytics-realtime/aidial_analytics_realtime/dial.py

Lines 45 to 49 in 492e8a2

	if isinstance(content_part, dict):
	if content_part.get("type") == "text" and (
	text := content_part.get("content")
	):
	ret.extend(text)

The typo content -> text was fixed.

Also, the empty strings are moved from the content.