Remove redundant nullable in dispatched column

Co-authored-by: John Davis <jdavcs@gmail.com>

lib/galaxy/model/__init__.py

@@ -2949,7 +2949,7 @@ class Notification(Base, Dictifiable, RepresentById):
         String(16), index=True, nullable=True
     )  # Defines the 'importance' of the notification ('info', 'warning', 'urgent', etc.). Used for filtering, highlight rendering, etc
     dispatched: Mapped[Boolean] = mapped_column(
-        Boolean, index=True, nullable=False, default=False
+        Boolean, index=True, default=False
     )  # Whether the notification has been dispatched to users via other channels
     # A bug in early 23.1 led to values being stored as json string, so we use this special type to process the result value twice.
     # content should always be a dict