Fix ipython#15011, Exception.text may be None (ipython#15012)

And add a test for it.

Author: Carreau

IPython/core/ultratb.py

@@ -338,8 +338,11 @@ def _format_exception_only(
                 if textline == "":
                     # sep 2025:
                     # textline = py3compat.cast_unicode(value.text, "utf-8")
-                    assert isinstance(value.text, str)
-                    textline = value.text
+                    if value.text is None:
+                        textline = ""
+                    else:
+                        assert isinstance(value.text, str)
+                        textline = value.text
 
                 if textline is not None:
                     i = 0

tests/test_ultratb.py

@@ -498,3 +498,10 @@ def eggs(f, g, z=globals()):
     except:
         handler(*sys.exc_info())
     buff.write("")
+
+
+def testSyntaxError():
+    cell = "raise SyntaxError()"
+    expected = "SyntaxError\n"
+    with tt.AssertPrints(expected):
+        ip.run_cell(cell)