Create the column nullable

There's no real point in ever making the column non-nullable, and doing so
breaks the sytests.

synapse/storage/schema/delta/50/make_event_content_nullable.py

@@ -60,34 +60,33 @@
 
 
 def run_create(cur, database_engine, *args, **kwargs):
+    pass
+
+
+def run_upgrade(cur, database_engine, *args, **kwargs):
     if isinstance(database_engine, PostgresEngine):
         cur.execute("""
             ALTER TABLE events ALTER COLUMN content DROP NOT NULL;
         """)
         return
 
     # sqlite is an arse about this. ref: https://www.sqlite.org/lang_altertable.html
-    cur.execute("PRAGMA schema_version")
-    (oldver,) = cur.fetchone()
 
     cur.execute("SELECT sql FROM sqlite_master WHERE tbl_name='events' AND type='table'")
     (oldsql,) = cur.fetchone()
+
     sql = oldsql.replace("content TEXT NOT NULL", "content TEXT")
     if sql == oldsql:
         raise Exception("Couldn't find null constraint to drop in %s" % oldsql)
 
     logger.info("Replacing definition of 'events' with: %s", sql)
 
+    cur.execute("PRAGMA schema_version")
+    (oldver,) = cur.fetchone()
     cur.execute("PRAGMA writable_schema=ON")
-
     cur.execute(
         "UPDATE sqlite_master SET sql=? WHERE tbl_name='events' AND type='table'",
         (sql, ),
     )
-
     cur.execute("PRAGMA schema_version=%i" % (oldver+1,))
     cur.execute("PRAGMA writable_schema=OFF")
-
-
-def run_upgrade(*args, **kwargs):
-    pass

synapse/storage/schema/full_schemas/16/im.sql

@@ -19,7 +19,12 @@ CREATE TABLE IF NOT EXISTS events(
     event_id TEXT NOT NULL,
     type TEXT NOT NULL,
     room_id TEXT NOT NULL,
-    content TEXT NOT NULL,
+
+    -- 'content' used to be created NULLable, but as of delta 50 we drop that constraint.
+    -- the hack we use to drop the constraint doesn't work for an in-memory sqlite
+    -- database, which breaks the sytests. Hence, we no longer make it nullable.
+    content TEXT,
+
     unrecognized_keys TEXT,
     processed BOOL NOT NULL,
     outlier BOOL NOT NULL,