Revert hynek#657

The changes introduced in hynek#657 are no longer needed since
_format_exception() is now only called if _figure_out_exc_info() detects
an actual exception

src/structlog/_frames.py

@@ -16,22 +16,12 @@
 from .typing import ExcInfo
 
 
-def is_missing_exc_info(exc_info: ExcInfo) -> bool:
-    """
-    Return True if exc_info is the missing value.
-    """
-    return exc_info == (None, None, None)  # type: ignore[comparison-overlap]
-
-
 def _format_exception(exc_info: ExcInfo) -> str:
     """
     Prettyprint an `exc_info` tuple.
 
     Shamelessly stolen from stdlib's logging module.
     """
-    if is_missing_exc_info(exc_info):
-        return "MISSING"
-
     sio = StringIO()
 
     traceback.print_exception(exc_info[0], exc_info[1], exc_info[2], None, sio)

src/structlog/tracebacks.py

@@ -22,8 +22,6 @@
 from types import ModuleType, TracebackType
 from typing import Any, Iterable, Sequence, Tuple, Union
 
-from ._frames import is_missing_exc_info
-
 
 try:
     import rich
@@ -414,8 +412,6 @@ def __init__(
         self.use_rich = use_rich
 
     def __call__(self, exc_info: ExcInfo) -> list[dict[str, Any]]:
-        if is_missing_exc_info(exc_info):
-            return []
         trace = extract(
             *exc_info,
             show_locals=self.show_locals,