Fix AttributeError in convert_osc when winterm is None

[worksbyfriday]

Summary

Fixes #407.

On non-Windows platforms, winterm is None. When write() receives text containing OSC title-setting sequences (e.g., \033]0;title\a) and strip=True, convert_osc() unconditionally calls winterm.set_title(), which crashes with:

AttributeError: 'NoneType' object has no attribute 'set_title'

Reproduction:

from colorama.ansitowin32 import AnsiToWin32
import io

stream = io.StringIO()
conv = AnsiToWin32(stream, convert=False, strip=True)
conv.write('\033]0;test title\a')  # crashes

Fix

Added a self.convert and winterm guard before calling winterm.set_title(), consistent with the existing pattern used in get_win32_calls() (line 132) and call_win32().

Test

Added test_osc_codes_without_winterm that verifies OSC sequences don't crash when winterm is None.