fix: initialize md_text in _parse_rss_type to prevent UnboundLocalError

[smedavarapu1]

Summary

_parse_rss_type only assigned md_text inside if channel_title:, so any RSS feed with no <title> in its <channel> caused an UnboundLocalError on the subsequent md_text += lines.

Fix

Initialize md_text = "" before the conditional block, and change the first assignment to += for consistency.

Reproduction

from markitdown.converters._rss_converter import RssConverter
import io
r = RssConverter()
xml = b'<?xml version="1.0"?><rss version="2.0"><channel></channel></rss>'
r.convert(io.BytesIO(xml), ...)  # UnboundLocalError

Tests

186 existing tests pass. The fix handles: empty channel, description-only channel, title-only channel, and title+description channel.