Since NULL = NULL is always FALSE, this query needs to use IS instead.
This should fix the flood of DeliveryReceipts in the DB.
See https://www.sqlite.org/lang_expr.html
> The IS and IS NOT operators work like = and != except when one or both
> of the operands are NULL. In this case, if both operands are NULL,
> then the IS operator evaluates to 1 (true) and the IS NOT operator
> evaluates to 0 (false). If one operand is NULL and the other is not,
> then the IS operator evaluates to 0 (false) and the IS NOT operator is
> 1 (true). It is not possible for an IS or IS NOT expression to
> evaluate to NULL.
This allows users to set a default realname used if the per-network
realname isn't set.
A new "user update" command is introduced and can be extended to edit
other user properties and other users in the future.
