Fix crash when traceback frame has tb_lineno=None

When formatting exceptions, Loguru calls linecache.getline() with the
line number from traceback frames. In some edge cases on Python 3.12+,
tb_lineno can be None (see CPython issues #139531, #89726), which
causes a TypeError in linecache.getline().

This fix guards against None lineno values by returning an empty source
string in that case, allowing the exception to be logged even if the
source line cannot be retrieved.

Fixes #1435

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: Mr-Neutr0n

loguru/_better_exceptions.py

@@ -208,7 +208,11 @@ def _extract_frames(self, tb, is_first, *, limit=None, from_decorator=False):
         def get_info(frame, lineno):
             filename = frame.f_code.co_filename
             function = frame.f_code.co_name
-            source = linecache.getline(filename, lineno).strip()
+            # Guard against None lineno (edge case on Python 3.12+, see CPython #139531)
+            if lineno is None:
+                source = ""
+            else:
+                source = linecache.getline(filename, lineno).strip()
             return filename, lineno, function, source
 
         infos = []