Print cause of exceptions even when they have no traceback

For example, exceptions raised by 'multiprocessing' may not include a
traceback. Instead, they include the traceback from another process
formatted as a string in the description of the exception.
It contains important information, so it should be printed by rich.

CONTRIBUTORS.md

@@ -43,6 +43,7 @@ The following people have contributed to the development of Rich:
 - [Nicolas Simonds](https://github.com/0xDEC0DE)
 - [Aaron Stephens](https://github.com/aaronst)
 - [Gabriele N. Tornetta](https://github.com/p403n1x87)
+- [Nils Vu](https://github.com/nilsvu)
 - [Arian Mollik Wasi](https://github.com/wasi-master)
 - [Handhika Yanuar Pratama](https://github.com/theDreamer911)
 - [za](https://github.com/za)

rich/traceback.py

@@ -389,19 +389,17 @@ class Traceback:
                     del stack.frames[:]
 
             cause = getattr(exc_value, "__cause__", None)
-            if cause and cause.__traceback__:
+            if cause:
                 exc_type = cause.__class__
                 exc_value = cause
+                # __traceback__ can be None, e.g. for exceptions raised by the
+                # 'multiprocessing' module
                 traceback = cause.__traceback__
                 is_cause = True
                 continue
 
             cause = exc_value.__context__
-            if (
-                cause
-                and cause.__traceback__
-                and not getattr(exc_value, "__suppress_context__", False)
-            ):
+            if cause and not getattr(exc_value, "__suppress_context__", False):
                 exc_type = cause.__class__
                 exc_value = cause
                 traceback = cause.__traceback__